Precious metal recovery using thiocyanate lixiviant

ABSTRACT

Precious metal-containing mineral material is subjected to an acidic thiocyanate leach to dissolve the precious metal as a precious metal-thiocyanate complex. A feed of the thiocyanate leach solution may include a large molar ratio of ferric iron to thiocyanate. Precious metal may be removed from pregnant thiocyanate leach solution, such as by transferring precious metal from precious metal-thiocyanate complex to precious metal-cyanide complex and then loading the precious metal-cyanide complex onto an adsorbent material. Remaining cyanide in the thiocyanate leach solution may be converted to thiocyanate for additional leaching of precious metal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims a priority benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/470,045 by Wan etal. entitled “PRECIOUS METAL RECOVERY USING THIOCYANATE LIXIVIANT” filedMay 12, 2003 and to U.S. Provisional Patent Application No. 60/460,795by Wan et al. entitled “PRECIOUS METAL RECOVERY USING THIOCYANATELIXIVIANT” filed Apr. 4, 2003, the entire contents of both of which areincorporated herein as if set forth herein in full.

FIELD OF THE INVENTION

[0002] The invention described herein relates to methods for use inmineral processing for precious metal recovery, and more specifically tothe use of thiocyanate lixiviants for gold recovery.

BACKGROUND OF THE INVENTION

[0003] A common technique for recovering gold from gold-bearing ores isto leach the gold into an aqueous cyanide leach solution in which thegold is solubilized as gold-cyanide complex. In some instances, the goldis leached directly from the ore or a concentrate prepared from the ore.This is the case for many oxide ores. In other instances, prior tocyanide leaching the ore or ore concentrate is pretreated to effect achemical change to enhance cyanide leach performance. For example,gold-bearing sulfide ores are often refractory to direct cyanideleaching. Therefore, prior to cyanide leaching, such refractory sulfideores, or sulfide concentrates prepared from such refractory sulfideores, are often subjected to an oxidative pretreatment to decomposesulfide minerals and thereby release gold for recovery during subsequentcyanide leaching.

[0004] The leach solution loaded with gold is often referred to as a“pregnant” leach solution. After the gold has been dissolved into acyanide leach solution, the gold is then removed from the pregnantcyanide leach solution. This is typically accomplished by contacting thepregnant cyanide leach solution with activated carbon granules underconditions conducive to adsorption of the gold-cyanide complex onto theactivated carbon granules. After the carbon granules are loaded withgold, the carbon granules are then separated from the now barren cyanideleach solution, which may be recycled to leach additional gold. Gold isremoved from the loaded carbon granules by stripping the gold from thecarbon granules using a suitable strip solution, such as for example ahot caustic solution. The gold is then removed from the strip solution,such as for example by electro-winning to prepare a crude metallicproduct called doré. The doré is subjected to further refining to highpurity gold.

[0005] Cyanide leaching can be conducted either in a heap operation orin a reactor. In a heap operation, barren cyanide leach solution feed isapplied to the surface of a heap of the mineral material to be treated.The cyanide leach solution percolates through the heap and leaches goldfrom the mineral material. Pregnant cyanide leach solution draining fromthe heap is collected and contacted with activated carbon to remove goldfrom the pregnant cyanide leach solution.

[0006] When cyanide leaching is conducted in a reactor, the mineralmaterial to be leached is slurried with the cyanide leach solution in areactor vessel or vessels for sufficient time for effective leaching ofthe gold. In a so-called carbon-in-pulp operation, following the cyanideleach, activated carbon is then contacted with the pulp in a series ofvessels, with the activated carbon advancing through the series ofvessels in a countercurrent fashion relative to advancement of the pulp.In a so-called carbon-in-leach operation, the leaching is conducted inthe presence of the activated carbon, so that cyanide leaching andadsorption onto activated carbon occur simultaneously in the samevessels.

[0007] The cyanide leach process is an industry standard that works wellin many situations. There are, however, situations when implementationof a cyanide leach is difficult or impractical.

[0008] One problematic situation involves the processing of refractorysulfide gold ores in which the gold is locked within one or more sulfidemineral from which the gold is generally not amenable to recovery bydirect cyanide leaching. As noted, these ores, or concentrates made fromsuch ores, are frequently subjected to an oxidative pretreatment priorto cyanide leaching. During the oxidative pretreatment, at least aportion of sulfide sulfur in the sulfide mineral is oxidized, resultingin decomposition of sulfide minerals and release of gold. The goldreleased from the sulfide minerals remains with the solids following theoxidative pretreatment, and the solids are then leached with a cyanideleach solution to dissolve the gold. One oxidative pretreatmenttechnique is bio-oxidation, in which sulfide sulfur in the ore orconcentrate is oxidized as a result of microbial activity. Anotheroxidative pretreatment technique is pressure oxidation, in which the oreor concentrate is subjected to oxygen gas at high temperature andpressure in an autoclave. Yet another oxidative pretreatment techniqueis oxidative roasting of the ore or concentrate.

[0009] A problem with cyanide leaching of the residual solids followingoxidative pretreatment is that the residual solids are often highlyacidic, while the cyanide leach must ordinarily be conducted at analkaline pH. As a consequence, it is necessary to neutralize the solidsprior to cyanide leaching. This neutralization typically requires addinglarge quantities of lime or some other neutralizing reagent to thesolids, and significantly adds to the expense and complexity of theoperation. In the case of bio-oxidation that has been performed in aheap, neutralizing the solids requires removing the heap followingbio-oxidation, neutralizing the oxidized solids by mixing the solidswith lime or some other neutralizing reagent, and then depositing a newheap of the neutralized mixture for the cyanide leach. Removing a heap,neutralizing solids and depositing a new heap following bio-oxidation tofacilitate cyanide leaching significantly add to the cost and complexityof the gold recovery operation. Also, even after the addition of aneutralizing agent, the solids typically still contain significantsulfide sulfur, the presence of which can complicate gold recoveryoperations. For example, some amount of sulfide sulfur may continue tooxidize during cyanide leaching operations, and even low levels of suchoxidation may cause significant material handling problems. One suchmaterial handling problem is that oxidized sulfur can react with calciumfrom the neutralizing agent (e.g., from the lime addition) to forminsoluble gypsum, that can plug pores in the heap, resulting inlocalized reductions in heap permeability during cyanide leaching. Asanother example, cyanide lixiviant may react directly with remainingsulfide sulfur, resulting in high consumption of cyanide.

[0010] Another problematic situation involves processing sulfide goldores that have only a moderate sulfide mineral content. As opposed torefractory sulfide gold ores, these moderately sulfuric ores typicallyhave a lower sulfide sulfur content and an appreciable portion of thegold is frequently recoverable by direct cyanide leaching. Cyanideleaching is, nevertheless, operationally difficult because these orestend to be highly acidic, and often produce significant quantities ofsulfuric acid as sulfide minerals oxidize during storage and duringcyanide leaching operations. The need to neutralize such ores forcyanide leaching presents a significant problem.

[0011] Yet another problematic situation involves processing eithersulfide or oxide gold ores that contain appreciable quantities of copperin a form that is susceptible to dissolving into the cyanide leachsolution along with the gold. The presence of significant quantities ofdissolved copper in the cyanide leach solution complicates gold recoveryand increases processing costs. Furthermore, it is necessary to destroycopper cyanide for disposal, further increasing processing costs.Although it is sometimes possible to preleach copper from the ore, suchas with a sulfuric acid solution, the preleached ore will still requireneutralization prior to cyanide leaching. Also, if the ore is beingprocessed in a heap operation, following the acidic preleach it isnecessary to remove the heap, neutralize the solids and deposit a newheap for the cyanide leach, presenting problems similar to the situationwith bio-oxidation of refractory sulfide ores, as discussed above.

[0012] Still a further problematic situation involves processing ofeither sulfide or oxide gold ores that contain appreciable quantities oforganic carbonaceous material that has an affinity to adsorb thegold-cyanide complex during cyanide leaching. Such refractorycarbonaceous ores are frequently referred to as “preg-robbing” ores,because available gold is “robbed” from the pregnant cyanide leachsolution by the organic carbonaceous material. Several pretreatmenttechniques have been proposed to reduce or eliminate the preg-robbingability of the organic carbonaceous material. These pretreatmenttechniques typically leave the ore in an acidic state requiringneutralization prior to cyanide leaching. As an alternative, thiosulfatelixiviants have been used to leach gold from such refractorycarbonaceous ores without first pretreating the ores to destroy thepreg-robbing ability of the organic carbonaceous material. The resultinggold-thiosulfate complex is less susceptible to being adsorbed onorganic carbonaceous material than gold-cyanide complex. As withcyanide, however, such thiosulfate leaching operations must generally beoperated at an alkaline pH, which can require significant neutralizationprior to the thiosulfate leach, depending upon the specific ore beingprocessed and the specific processing operation being employed.

[0013] In addition to the foregoing, there continues to be increasedregulatory restrictions placed on the use of cyanide for gold leachingoperations. There has, therefore, been interest in the gold miningindustry to identify alternative processes for leaching gold that uselixiviants other than cyanide. For example, the potential use ofthiourea and thiosulfate lixiviants has received considerable attention.The use of thiourea, however, is typically not practical due to highthiourea consumption caused by a high susceptibility of thiourea tooxidative degradation. Greater success has been achieved with the use ofthiosulfate lixiviants, but, as noted, thiosulfate leaching operationsgenerally must be conducted at an alkaline pH, presenting the sametechnical problems in many situations as noted previously with respectto cyanide leaching. Moreover, removal of gold from pregnant thiosulfateleach solutions is considerably more difficult than gold removal frompregnant cyanide leach solutions, because gold-thiosulfate complex doesnot readily adsorb onto activated carbon granules. Still otherlixiviants have been suggested as alternatives for cyanide, but have notbeen investigated to a large extent, and practical implementation hasbeen uncertain.

SUMMARY OF THE INVENTION

[0014] With the present invention, it has been found that with carefulcontrol of leach conditions, thiocyanate leaching of precious metal, andparticularly gold, from a precious metal-containing mineral material maybe effective using only a very low concentration of dissolvedthiocyanate in an acidic thiocyanate leach solution. Moreover, it hasbeen found that such an acidic thiocyanate leach may be advantageouslycombined with acidic pretreatment operations for processing some oresand concentrates in preparation for recovering precious metal. Suchacidic pretreatment operations may include, for example, oxidativepretreatment of refractory sulfide ores and concentrates to releaseprecious metal from sulfide minerals or acidic pre-leaching of ores toselectively preleach soluble copper, or some other soluble component,prior to precious metal recovery.

[0015] For enhanced performance of the acidic thiocyanate leach,properties of the thiocyanate leach solution, and especially in the feedof the thiocyanate leach solution fed to the thiocyanate leachoperation, are carefully controlled. The thiocyanate leach solution istypically an acidic aqueous solution, and for enhanced performance thepH of the thiocyanate leach solution is controlled within a narrow rangeof pH and the thiocyanate leach solution contains dissolved ferric ironat a high concentration relative to the concentration of dissolvedthiocyanate. By dissolved thiocyanate, it is meant that the leachsolution includes dissolved species including the thiocyanate chemicalgroup, SCN. The primary soluble thiocyanate specie will generally be thethiocyanate ion SCN⁻, although other soluble thiocyanate species mayalso be present, such as for example tri-thiocyanate (SCN)₃ ⁻ andthiocyanogen (SCN)₂. The dissolved thiocyanate may include uncomplexedthiocyanate species and/or thiocyanate species complexed with one ormore metals, and often complexed with ferric iron. Table 1 shows someexemplary iron-thiocyanate complex species and stability constants thathave been reported for the species (Barbosa-Filho, O., and Monhemius, A.J., Leaching of gold in thiocyanate solutions—Part I: Chemistry andthermodynamics, Transactions of the Institute of Mining and Metallurgy(Section C), 1994, Vol. 103, C117-125). TABLE 1 Iron Ion ComplexStability Constant (At 25° C.) Fe²⁺ (ferrous iron) FeSCN⁺ 2.04 × 10¹Fe³⁺ (ferric iron) FeSCN²⁺ 1.05 × 10³ Fe(SCN)₂ ⁺ 2.00 × 10⁵ Fe(SCN)₄ ⁻3.31 × 10⁵ Fe(SCN)₅ ²⁻ 1.58 × 10⁶ Fe(SCN)₆ ³⁻ 1.26 × 10⁶

[0016] Dissolved thiocyanate in the leach solution is capable ofcomplexing with precious metal to solubilize precious metal in the leachsolution. Table 2 shows examples of some gold-thiocyanate complexspecies and stability constants that have been reported for the complexspecies (Barbosa-Filho, O., and Monhemius, A. J., Leaching of gold inthiocyanate solutions—Part I: Chemistry and thermodynamics, Transactionsof the Institute of Mining and Metallurgy (Section C), 1994, Vol. 103,C117-125). Under conditions of thiocyanate leaching according to thepresent invention, di-thiocyano-aurous (Au(SCN₂)⁻) andtetrathiocyano-auric (Au(SCN)₄ ⁻) complexes appear to be the mostimportant gold-thiocyanate complex species for dissolution of the goldduring the thiocyanate leach. TABLE 2 Gold Ion Complex StabilityConstant (At 25° C.) Au⁺ Au(SCN)_(aq) 1.86 × 10¹⁵ Au(SCN)₂ ⁻ 1.45 × 10¹⁹Au³⁺ Au(SCN)₄ ⁻ 4.57 × 10⁴³ Au(SCN)₅ ²⁻ 4.17 × 10⁴³ Au(SCN)₆ ³⁻ 4.68 ×10⁴³

[0017] For enhanced performance, the pH of the feed of the thiocyanateleach solution, as supplied to the thiocyanate leach, should be in anacidic range having a lower limit of pH 0.75, preferably pH 1 and morepreferably pH 1.5 and having an upper limit of pH 3.5, preferably pH 3and more preferably pH 2.5. One preferred range for the feed of thethiocyanate leach solution is from pH 1 to pH 3, with a pH of from 1.5to pH 2.5 being more preferred. A pH of pH 2 is particularly preferredfor the feed of the thiocyanate leach solution. In one possible processenhancement, the thiocyanate leach solution may be maintained within thenoted acidic pH ranges throughout the thiocyanate leach, and preferablyalso during subsequent precious metal recovery operations. In anotherpossible process enhancement, the feed of the thiocyanate leach solutionmay be carefully prepared to contain a high concentration of dissolvedferric iron relative to the concentration of dissolved thiocyanate. Forthis enhancement, the feed of the thiocyanate leach solution preferablyhas a molar ratio of dissolved ferric iron to dissolved thiocyanate(such ratio being sometimes referred to herein as [Fe³⁺]/[SCN]) of atleast 2, more preferably at least 4, even more preferably at least 7,still more preferably at least 8 and most preferably at least 10. As yeta further possible enhancement, the molar ratio of the dissolved ferriciron to the dissolved thiocyanate may be maintained at a level that isnot larger than 20. The molar ratio of the dissolved ferric iron to thedissolved thiocyanate may be determined by dividing the molarconcentration of the dissolved ferric iron by the molar concentrationsof the dissolved thiocyanate. By molar concentration, it is meant thegram-moles (referred to herein simply as moles) of dissolved ferric ironor dissolved thiocyanate, as the case may be, per liter of solution(molar concentrations sometimes being designated herein with theabbreviated symbol “M”). As used herein, concentration refers to molarconcentration unless specifically noted otherwise. As used herein, aconcentration denoted as “ppm” refers to parts per million parts on aweight basis. As will be appreciated, the ratio of the molarconcentrations of dissolved ferric iron to dissolved thiocyanate is alsoequal to the ratio of the total moles of the dissolved ferric iron tothe total moles of dissolved thiocyanate in the leach solution. Theratio of molar concentrations of components and the ratio of total molesof the components are each often referred to herein simply as a “molarratio” of the components.

[0018] Maintaining a high concentration of dissolved ferric ironrelative to dissolved thiocyanate significantly improves the kinetics ofgold dissolution. Not to be bound by theory, but to aid in understandingof the invention, the improved kinetics are believed to be related towhat has been referred to as an “auto-reduction” process, in which thethiocyanate ion SCN⁻ is oxidized by the spontaneous reduction of ferriciron to ferrous iron. Intermediate thiocyanate species produced as aresult of this auto-reduction process are believed to be important forspeeding the oxidation and dissolution of gold during the thiocyanateleach.

[0019] As a further enhancement, the concentration of dissolvedthiocyanate in the thiocyanate leach solution may be maintained at avery low concentration. For preferred operation, the concentration ofthe dissolved thiocyanate in feed of the thiocyanate leach solution isno larger than 0.03 M, more preferably no larger than 0.02 M and evenmore preferably no larger than 0.01 M. The concentration of dissolvedthiocyanate in the feed of the thiocyanate leach solution will, however,typically be at least 0.0001 M, preferably at least 0.001 M, morepreferably at least 0.002 M, and often at least 0.005 M. Also, as afurther enhancement, the dissolved thiocyanate concentration in thethiocyanate leach solution may be preferably maintained at the noted lowconcentrations throughout the thiocyanate leach, and preferably alsoduring precious metal recovery operations. As used herein, theconcentration of dissolved thiocyanate is determined with respect to theSCN chemical group. In effect, all thiocyanate species are assumed to bein the form of the thiocyanate ion SCN⁻ for the purpose of determiningthe concentration of dissolved thiocyanate. For example, SCN⁻ counts asone mole of dissolved thiocyanate, whereas a mole of (SCN)₂ counts astwo moles of dissolved thiocyanate and a mole of (SCN)₃ ⁻ counts asthree moles of dissolved thiocyanate.

[0020] As noted previously, the concentration of dissolved ferric ironin the thiocyanate leach solution should preferably be significantlylarger than the concentration of dissolved thiocyanate. In one preferredimplementation, the concentration of dissolved ferric iron in the feedof the thiocyanate leach solution may be at least 0.05 M, and preferablyat least 0.1 M, although in some instances it may be desirable for theconcentration of the dissolved ferric iron to be at least 0.2 M or evenhigher. In one preferred implementation, the concentration of dissolvedferric iron in the feed of the thiocyanate leach solution is in a rangeof from 0.05 M to 0.3 M and more preferably in a range of from 0.05 M to0.2 M.

[0021] The thiocyanate leach may be conducted at any desiredtemperature, providing that the temperature is not detrimental todissolution of gold in the form of gold-thiocyanate complex. Typically,the thiocyanate leach is conducted at a temperature in a range of from15° C. to 50° C.

[0022] The precious metal-containing mineral material processedaccording to the invention may include, for example, one or more of thefollowing: precious metal-bearing ore, precious metal-containingconcentrate (such as for example produced from processing an ore byflotation or other concentration techniques), and precious metal-bearingsolid residue from prior mineral processing operations (such as forexample solids resulting from prior oxidative pretreatment of a sulfideore or concentrate or tailings resulting from prior milling operationsthat still contain precious metal). Moreover, the preciousmetal-containing mineral material may include a combination ofmaterials, such as for example a combination of two or more of thefollowing: ore, concentrate and solid residue from prior mineralprocessing operations.

[0023] In one aspect, the invention involves a method for processing aprecious metal-containing mineral material in which the mineral materialis subjected to an acidic thiocyanate leach, preferably with carefulcontrol of properties of the feed of the thiocyanate leach solution, andparticularly with respect to pH, thiocyanate concentration, ferric ironconcentration and the molar ratio of thiocyanate to ferric iron. Thefeed of the thiocyanate leach solution may be prepared by conditioningthe thiocyanate leach solution, with the conditioning comprisingrecycling barren effluent of the thiocyanate leach solution followingprecious metal recovery and increasing the concentration of ferric ironin the thiocyanate leach solution relative to the concentration offerric iron in the recycled effluent of the thiocyanate leach solution.During the conditioning, increasing the concentration of the ferric ironmay involve, for example, adding to the thiocyanate leach solution aferric iron-rich acidic effluent liquid from an oxidative pretreatmentoperation (such as bio-oxidation or pressure oxidation), adding a ferriciron-containing reagent to the thiocyanate leach solution, and/or byoxidizing ferrous iron in the thiocyanate leach solution to the ferricform.

[0024] In one aspect, the invention involves a method comprising acidicpretreatment of a mineral material feed prior to a thiocyanate leach. Inone implementation the mineral material feed comprises preg-robbingorganic carbon, and the acidic pretreatment involves oxidative treatmentto decompose and/or passivate the organic carbon to reduce thepreg-robbing capability of the mineral material. In anotherimplementation, the mineral material feed comprises precious metallocked in sulfide minerals, such as might be the situation withrefractory sulfide ores and concentrates, and the acidic pretreatmentinvolves oxidative pretreatment to decompose sulfide minerals to releaseprecious metal prior to the thiocyanate leach. In one implementation,oxidative pretreatment may include, for example, bio-oxidation orpressure oxidation of the mineral material feed. In anotherimplementation, the mineral material feed may include a nonferrous,nonprecious metal (such as for example one or more of copper, nickel,zinc and lead) in sufficient quantity for economic recovery, and theacidic pretreatment may involve leaching the nonferrous nonpreciousmetal from the mineral material feed prior to the thiocyanate leach toextract precious metal. Leaching of the nonferrous nonprecious metal mayalso involve decomposition of sulfide minerals, such as for example,during bio-oxidation or pressure oxidation pretreatment to decomposesulfide minerals. In one implementation, the acidic pretreatment mayinvolve acidic leaching of a component from the mineral material feedthat would otherwise interfere with or complicate precious metalrecovery using the thiocyanate leach. For example, when the mineralmaterial feed includes appreciable soluble copper, the soluble coppermay be removed in an acidic preleach, such as using an acidic sulfateleach solution. This implementation may be used, for example, to removenuisance quantities of soluble copper from the mineral material feedprior to the thiocyanate leach of the precious metals, to recoverby-product copper from precious metal ores or concentrates, or to permitby product precious metal recovery following copper recovery from copperores or concentrates, such as in a copper dump leach operations.

[0025] When implementation of the invention involves acidic pretreatmentprior to the acidic thiocyanate leach, one of both of the operations maybe performed in a reactor, such as in a tank, vat or pressure vessel,depending upon the circumstances. In one preferred implementation,however, the acidic pretreatment and the thiocyanate pretreatment areperformed sequentially on a heap initially including the mineralmaterial feed. This is possible with the present invention, because bothoperations are performed at an acidic pH, and it is not necessary toremove the heap after the acidic pretreatment and redeposit a new heap,as is the case with cyanide leaching operations when solids must beneutralized in preparation for the cyanide leach. With respect tobio-oxidation pretreatment of a refractory sulfide material, in oneimplementation the level of oxidation of sulfide sulfur during thebio-oxidation pretreatment may be lower than would be required prior tocyanide leaching. This is because additional oxidation of easilyoxidized sulfide minerals may be accommodated in the acidic conditionsof the thiocyanate leach, whereas such additional oxidation would bedetrimental to an alkaline cyanide leach.

[0026] In another aspect, the invention involves direct leachingprecious metal from a mildly refractory sulfide material using theacidic thiocyanate leach. The thiocyanate leach is well suited toprocessing such naturally acidic materials. In one implementation, thethiocyanate leach may be preceded by an acidic pretreatment wash.

[0027] In one aspect, the invention involves recovery of precious metalfrom pregnant thiocyanate leach solutions. In one implementation, acidicpregnant thiocyanate leach solution is contacted with an organicextractant phase for transfer of dissolved precious metal from theaqueous thiocyanate leach solution into the organic extractant phase. Inanother implementation, precious metal in the acidic thiocyanate leachsolution is transferred from precious metal-thiocyanate complex to acomplex with a different complexing agent, preferably cyanide, to form adifferent precious metal complex, preferably precious metal-cyanidecomplex, in the thiocyanate leach solution. The precious metal-cyanidecomplex may then be recovered from the thiocyanate leach solution, suchas by loading precious metal onto an adsorption material, such as ionexchange resin or activated carbon. To effect the complex transfer, asmall quantity of dissolved cyanide may be introduced into the acidicpregnant thiocyanate leach solution. In one implementation, followingremoval of the precious metal, residual cyanide in the thiocyanate leachsolution is converted to thiocyanate.

[0028] In one aspect, the invention involves preparation and/orconditioning of acidic thiocyanate leach solutions involving convertingdissolved cyanide to dissolved thiocyanate in n acidic aqueous liquid.This implementation may be used, for example, to initially prepare athiocyanate solution or to compensate for thiocyanate losses duringthiocyanate leaching operations.

[0029] Additional disclosure concerning these and other aspects of theinvention are provided in the detailed description presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving thiocyanateleaching.

[0031]FIG. 2 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving thiocyanateleaching and recycling of thiocyanate leach solution following preciousmetal recovery.

[0032]FIG. 3 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving oxidativepretreatment prior to thiocyanate leaching.

[0033]FIG. 4 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving pressure oxidationpretreatment prior to thiocyanate leaching.

[0034]FIG. 5 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving pressure oxidationpretreatment prior to thiocyanate leaching, with the pressure oxidationpretreatment including general steps of autoclave processing and solidsconditioning.

[0035]FIG. 6 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving bio-oxidationpretreatment prior to thiocyanate leaching.

[0036]FIG. 7 is a schematic depicting one embodiment of animplementation for agglomerating particulate mineral material for heapprocessing with the invention.

[0037]FIG. 8 is a generalized process schematic of one embodiment of animplementation of the present invention involving simultaneousbio-oxidation and thiocyanate leaching of different heaps.

[0038]FIG. 9 is a generalized process block diagram of one embodiment ofan implementation of the present invention involving acid wash of a feedmaterial prior to thiocyanate leaching.

[0039]FIG. 10 is a generalized process block diagram of one embodimentof an implementation of the present invention involving leaching ofcopper from a feed material prior to thiocyanate leaching.

[0040]FIG. 11 is a generalized process block diagram of one embodimentof an implementation of the present invention involving solventextraction recovery of precious metal following thiocyanate leaching,and recycling of the thiocyanate leach solution for use to prepare feedof the thiocyanate leach solution for supply to the thiocyanateleaching.

[0041]FIG. 12 is a generalized process block diagram of one embodimentof an implementation of the present invention involving complex transferduring precious metal recovery operations, and recycling of thethiocyanate leach solution for use to prepare feed of the thiocyanateleach solution for supply to the thiocyanate leaching.

[0042]FIG. 13 is a generalized process block diagram of one embodimentof an implementation of the present invention involving conversion ofcyanide to thiocyanate for use in thiocyanate leaching.

[0043]FIG. 14 is a plot of gold extraction vs weight ratio ofthiocyanate leach solution to ore for cyanide leach tests presented inExample 1.

[0044]FIG. 15 is a plot of gold extraction vs thiocyanate concentrationin thiocyanate leach solutions for thiocyanate leach tests presented inExample 2.

[0045]FIG. 16 is a plot of gold extraction vs thiocyanate concentrationin thiocyanate leach solutions for thiocyanate leach tests presented inExample 3.

[0046]FIG. 17 is a plot of gold extraction vs ferric iron concentrationin thiocyanate leach solutions for thiocyanate leach tests presented inExample 3.

[0047]FIG. 18 is a bar plot of gold extraction and a line plot ofthiocyanate consumption for thiocyanate leach tests presented in Example4.

[0048]FIG. 19 is a plot of gold extraction vs leach duration forthiocyanate and cyanide leach tests presented in Example 5.

[0049]FIG. 20 is a plot of thiocyanate or cyanide reagent consumption vsleach duration for thiocyanate and cyanide leach tests presented inExample 5.

[0050]FIG. 21 is a plot of gold loading on ion exchange resin vs goldconcentration in pregnant thiocyanate solutions for ion exchangerecovery tests presented in Example 9.

[0051]FIG. 22 is a plot of gold concentration in thiocyanate solutionsvs time during ion exchange precious metal recovery tests presented inExample 9.

[0052]FIG. 23 is a plot of gold recovery from pregnant thiocyanatesolutions vs time during ion exchange precious metal recovery testspresented in Example 9.

[0053]FIG. 24 is a generalized process block diagram of one embodimentof an implementation of the present invention involving processing of amaterial including recoverable nonferrous base metal and precious metal,with pretreatment leaching associated with recovery of nonferrous basemetal followed by thiocyanate leaching associated with recovery ofprecious metal.

DETAILED DESCRIPTION OF THE INVENTION

[0054] As used herein, “precious metal” includes gold and/or silver. Theinvention is described and illustrated herein primarily with referenceto processing and recovery of gold, but the same principles apply alsoto processing and recovery of silver, with or without also processingand recovering gold.

[0055] Referring to FIG. 1, a generalized process block diagram is shownfor one implementation of an acidic thiocyanate leach according to thepresent invention. As shown in FIG. 1, a particulate, gold-bearingmineral material 102 is subjected to a thiocyanate leach 104 underacidic conditions. During the thiocyanate leach 104, gold is leachedfrom the mineral material 102 into a thiocyanate leach solution. Feed ofthe thiocyanate leach solution 106 is fed to the thiocyanate leach 104to contact the mineral material 102. Pregnant thiocyanate leach solution108 containing dissolved gold in the form of gold-thiocyanate complex isremoved from the thiocyanate leach 104, and may be further processed toremove the dissolved gold. Residual solids 110 that are depleted in goldmay be subjected to additional metal recovery operations or to furthertreatment for disposal in an appropriate manner. By gold-thiocyanatecomplex, it is meant any and all soluble gold-thiocyanate species thatmay be present in the pregnant thiocyanate leach solution 108, such asfor example any of the species listed in Table 2.

[0056] Prior to being fed to the thiocyanate leach, the thiocyanateleach solution is subjected to leach solution conditioning 112. Duringthe leach solution conditioning 112, the properties of the thiocyanateleach solution are adjusted to the properties desired for the feed ofthiocyanate leach solution 106 that is supplied to the thiocyanate leach104. These properties of the thiocyanate leach solution that may beadjusted during the leach solution conditioning include, but are notlimited to, concentrations and molar ratio of dissolved thiocyanate anddissolved ferric iron, pH and temperature. The leach solutionconditioning 112 could include, for example, one or more of adjustingpH, adjusting temperature, adjusting dissolved ferric ironconcentration, adjusting dissolved thiocyanate concentration,precipitating undesirable components, bleeding liquid and adding liquid,to effect desired adjustment of the properties of the thiocyanate leachsolution to prepare the feed of thiocyanate leach solution 106.

[0057] In one preferred implementation of the invention, during theleach solution conditioning 112 the concentrations of the dissolvedthiocyanate and dissolved ferric iron are adjusted, as well as the molarratio of dissolved ferric iron to dissolved thiocyanate. Also, dependingupon the circumstances, undesirable components may be precipitated toprevent excessive build-up of the components in the thiocyanate leachstream and to adjust the pH to be within a desired range. Becauseleaching gold with thiocyanate involves reduction of at least some ofthe ferric iron to ferrous iron, the leach solution conditioning 112will typically involve increasing the concentration of ferric iron inthe thiocyanate leach solution. Also, at least some fresh thiocyanatewill typically be added during the leach solution conditioning 112 tocompensate for thiocyanate losses. Increasing the concentration offerric iron in the thiocyanate leach solution during the leach solutionconditioning 112 may be accomplished by a variety of techniques. Forexample, a reagent containing ferric iron that is soluble in thethiocyanate leach solution may be added to the thiocyanate leachsolution.

[0058] In one preferred variation, one or more oxidant reagents areintroduced into the thiocyanate leach solution to cause ferrous ironalready dissolved in the thiocyanate leach solution to be oxidized toferric iron. By “oxidant” or “oxidant reagent” it is meant a substancethat alone or in combination with another substance causes oxidation ofdissolved ferrous iron to dissolved ferric iron in the thiocyanate leachsolution. Such oxidant reagent(s) may be introduced into the thiocyanateleach solution during any convenient stage of processing. For example,in the implementation shown in FIG. 1, oxidant(s) may be introduced intothe thiocyanate leach solution during the thiocyanate leach 104 and/orduring the leach solution conditioning 112. When oxidant(s) areintroduced into the thiocyanate leach solution during gold leaching,such as during the thiocyanate leach 104, the oxidant(s) shouldpreferably be selected so as not to interfere with, and more preferablyto enhance, the gold leaching rate, and so as not to affect thiocyanatedegradation. Also, in this situation, oxidizing potential and pHconditions in the thiocyanate leach solution should be controlled withappropriate reagent additions, as appropriate under the circumstances.When oxidant(s) are introduced into an effluent or recycled thiocyanateleach solution following precious metal recovery operations, theoxidant(s) should preferably provide the desired oxidation to produceferric iron, but more preferably without causing significant destructionof thiocyanate dissolved in the solution. Some nonlimiting examples ofpotential oxidant reagents that may be used alone, or in combinationwith other chemically compatible reagents, include Caro's acid(persulfuric acid), persulfates (such as for example ammonium persulfateand alkali metal persulfates, such as potassium persulfate and sodiumpersulfate), peroxides (such as for example inorganic peroxides, such ashydrogen peroxide and alkali metal peroxides, and organic peroxides),manganese dioxide, ozone, halogens (such as for example chlorine, iodineand bromine) and hypochlorites (such as for example sodiumhypochlorite). As noted, one or more of these oxidant reagents may beused alone or with other chemically compatible reagents. Also, multipleof the oxidant reagents may be used together to the extent that themultiple oxidant reagents are chemically compatible in the particularsystem. An example of an oxidant reagent that is preferably used withone or more other reagent is oxygen gas (such as for example provided inair or in a purified oxygen gas composition). In one preferredvariation, oxygen gas is used in combination with at least a secondreagent to oxidize ferrous iron to ferric iron in the thiocyanate leachsolution. The oxygen gas may be introduced into the thiocyanate leachsolution for example, by sparging air or a stream of purified oxygen gasinto the thiocyanate leach solution. The second reagent is preferably,sulfur dioxide, a bisulfite (such as for example an alkali metalbisulfite or ammonium bisulfite) or a metabisulfite (such as for examplesodium metabisulfite or potassium metabisulfite). A copper or othercatalyst, such as copper sulfate, may also be used with the oxygen andthe second reagent for improved performance. In the case of sulfurdioxide as the second reagent, the sulfur dioxide may be introduced bysparging sulfur dioxide gas into the thiocyanate leach solution, eithertogether with or separate from the oxygen gas. In the case of abisulfite or metabisulfite as the second reagent, the second reagent maybe introduced into the thiocyanate leach solution as a solid thatdissolves in the thiocyanate leach solution or the second reagent may bepredissolved in a concentrated reagent solution.

[0059] In another variation, the leach solution conditioning 112includes oxidizing dissolved ferrous iron to dissolved ferric iron bythe action of microorganisms under acidic conditions, and preferably ata pH in a range suitable for use during the thiocyanate leach 112. Theoxidation could be performed on all or a portion of the thiocyanateleach solution in a separate bioreactor. The bioreactor may be, forexample, a separate heap or heaps containing the microorganisms on rockor some other support. As another example, the bioreactor may be one ormore tanks or vats containing the microorganisms on rock or some othersupport. Preferably, the support on which the microorganisms aredisposed is highly porous and permeable. In this variation, it isimportant that the thiocyanate not be toxic to the microorganisms, suchas for example microorganisms disclosed in U.S. Pat. No. 6,379,919entitled “Method of Isolating Thiocyanate Resistant Bacteria”, theentire contents of which are incorporated by reference herein as if setforth herein in full.

[0060] As yet a further variation, the leach solution conditioning 112comprises oxidation of ferrous iron to ferric iron in all or a portionof the thiocyanate leach solution is accomplished by passing all or aportion of the solution through an electro-chemical cell while applyinga sufficient electrical potential across the cell to cause dissolvedferrous iron to oxidize to dissolved ferric iron.

[0061] In a preferred implementation of the invention, thiocyanate leachsolution is recycled from gold recovery operations for reuse foradditional thiocyanate leaching. Referring now to FIG. 2, a generalizedprocess block diagram is shown of one implementation of the invention inwhich thiocyanate leach solution is recycled following gold recovery forreuse in the thiocyanate leaching operation. Reference numerals are thesame as those used in FIG. 1, except as noted. As shown in FIG. 2, thepregnant leach solution 108 is sent to gold recovery 118, where apurified gold product 120, such as for example dore, is produced. Barreneffluent of the thiocyanate leach solution 122, including all or aportion of the thiocyanate leach solution from which the gold has beenremoved during the gold recovery 118, is recycled to the leach solutionconditioning 112 for use to prepare the feed of thiocyanate leachsolution 106 for additional thiocyanate leaching.

[0062] The thiocyanate leaching of the invention may be used to processa variety of precious metal-containing mineral materials. For example,the mineral material could comprise an oxide ore, a sulfide ore,concentrate prepared from an oxide ore or a sulfide ore, some otherprecious metal-containing solid product produced during prior mineralprocessing operations, or a combination including any number of theforegoing in any proportions. The present invention, however, isparticularly well suited for processing mineral materials for which itwould be beneficial to leach the precious metal at an acidic pH, such asfor example naturally acidic ores, acidic concentrates or acidic solidresidues resulting from prior oxidative pretreatment.

[0063] In one embodiment, the invention is directed to recovery ofprecious metal from precious metal-bearing mineral material feedincluding precious metal contained in one or more sulfide mineral. Sucha mineral material feed could include, for example, a sulfide gold oreand/or gold-bearing sulfide concentrate prepared from a sulfide goldore, such as by flotation. Sulfide gold ores that are not amenable togold recovery by direct cyanide leaching because gold is bound insulfide mineralization are typically referred to as “refractory” sulfidegold ores. Such refractory sulfide gold ores typically have a highsulfide sulfur content, usually at least 2 weight percent sulfide sulfurand frequently at least 3 weight percent or more sulfide sulfur, buttypically less than 10 weight percent sulfide sulfur. Some ores containlower amounts of sulfide sulfur and may be only mildly refractory due tothe sulfide mineralization, with a significant amount of the gold beingrecoverable by direct cyanidation. These mildly refractory ores may havea significant gold-bearing oxide mineral component in addition to agold-bearing sulfide mineral component, in which case the ores aresometimes referred to as transition ores. Also, such mildly refractoryores may result from stockpiling for a significant time ores thatoriginally contained a higher level of sulfide sulfur, becauseappreciable sulfide sulfur oxidation may occur naturally during extendedstockpiling, such as due to the action of naturally-occurring bacteria.Mildly refractory sulfide ores typically include less than 2 weightpercent sulfide sulfur, and more typically include no more than 1.5weight percent sulfide sulfur, but also typically include at least 0.5weight percent sulfide sulfur.

[0064] The present invention may be used to treat refractory sulfidegold ores, mildly refractory sulfide gold ores, and/or sulfideconcentrates prepared from one or more of any such ores. As will beappreciated, a sulfide concentrate will contain a higher sulfide sulfurcontent than the ore(s) from which the concentrate is prepared. Thesulfide sulfur content of the sulfide concentrate is often at leasttwice as large and more often several times as large as the sulfidesulfur content of the ore materials from which the concentrate isprepared. The present invention may also be used to treat refractorygold ores and concentrates comprising significant gold not amenable torecovery by direct cyanide leaching for reasons other than or inaddition to gold being bound in sulfide mineralization, such as forexample because of the presence of pregrobbing organic carbon.

[0065] Referring now to FIG. 3, a generalized process block diagram isshown for one implementation of the present invention for processingrefractory gold ores, concentrates prepared from such refractory goldores and/or other gold-bearing refractory mineral material comprisinggold that is not amenable to recovery by direct cyanide leaching.Reference numerals are the same as those used in FIGS. 1 and 2, exceptas noted. As shown in FIG. 3, a refractory mineral material feed 128,typically in particulate form, is subjected to oxidative pretreatment130. The refractory mineral material feed 128 could be refractory due toone or multiple characteristics of the mineral material feed. Forexample, the refractory mineral material feed 128 could be refractorybecause it comprises significant gold that is bound in sulfide mineralsnot amenable to gold recovery by direct cyanide leaching (refractorysulfide mineral material) and/or because the refractory mineral materialfeed 128 comprises pregrobbing organic carbon (refractory carbonaceousmineral material). A refractory sulfide mineral material feed maycomprise preg-robbing organic carbon in addition to refractory sulfidemineral material. Likewise, a refractory carbonaceous mineral materialfeed may comprise refractory sulfide mineral material in addition to thepreg-robbing organic carbon.

[0066] During the oxidative pretreatment 130, chemical properties of therefractory mineral material feed 128 are altered in an oxidizingenvironment to reduce the refractory nature of the mineral material,thereby permitting recovery of gold that is not amenable to recovery bydirect cyanide leaching of the refractory mineral material feed 128. Inthe situation where the refractory mineral material feed 128 comprisesrefractory sulfide mineral material, during the oxidative pretreatment130 at least a portion of sulfide sulfur of sulfide minerals in therefractory sulfide mineral material is oxidized, and at least a portionof the sulfide minerals are thereby decomposed to release gold from thesulfide minerals. In the situation where the refractory mineral materialfeed 128 comprises refractory carbonaceous mineral material, during theoxidative pretreatment 130 the preg-robbing capability of organic carbonis reduced, such as by decomposition and/or passivation of thepreg-robbing organic carbon. In the situation where the refractorymineral material feed 128 includes both refractory sulfide mineralmaterial and refractory carbonaceous mineral material, during theoxidative pretreatment 130 at least a portion of sulfide sulfur ofsulfide minerals in the refractory sulfide mineral material is oxidizedto release gold from the sulfide minerals, or the preg-robbingcapability of the preg-robbing organic carbon is reduced, or both. Themineral material 102 that is subjected to the thiocyanate leach 104includes solid residue resulting from the oxidative pretreatment 130.Such solid residue will typically be highly acidic.

[0067] An optional feature with the implementation as shown in FIG. 3when the oxidative pretreatment generates significant dissolved ferriciron is to use during the leach solution conditioning 112 an acidiceffluent liquid 132 that is produced during the oxidative pretreatment130 that contains a high concentration of dissolved ferric iron. Theacidic effluent liquid 132, when used, will typically involve only aportion of liquid effluent produced during the oxidative pretreatment130. During the leach solution conditioning 112, the acidic effluentliquid 132 may be added to the thiocyanate leach solution to increasethe concentration of dissolved ferric iron in the thiocyanate leachsolution to prepare the feed of thiocyanate leach solution 106. Tomaintain the appropriate liquid volume of thiocyanate leach solution, aportion of the thiocyanate liquid may be bled or otherwise removed fromthe thiocyanate leach solution during the leach solution conditioning112. For example, water could be removed, such as by bleeding liquid orby evaporation, before or after addition of the acidic effluent liquid132, but preferably before. For the implementation of the invention asshown in FIG. 3 with the use of the acidic effluent liquid 132, theoxidative pretreatment 130 is of a type to produce the acidic effluentliquid 132 with a high concentration of dissolved ferric iron.

[0068] When used, the acidic effluent liquid 132 should includedissolved ferric iron at a concentration that is higher, and preferablymuch higher, than the concentration of dissolved ferric iron in the feedof thiocyanate leach solution 106. Examples of preferred operations forthe oxidative pretreatment 130 are pressure oxidation and bio-oxidation.

[0069] Preferably the concentration of dissolved ferric iron in theacidic effluent liquid 132 is at least twice as large (and morepreferably at least four times as large) as the concentration ofdissolved ferric iron in the feed of thiocyanate leach solution 106.Also, the acidic effluent liquid 132 may be substantially as producedduring the oxidative pretreatment 132, or may result from treatmentfollowing production in the oxidative pretreatment 132. For example, thepH of the acidic effluent liquid 132 may be adjusted up or down asdesired prior to being mixed with the thiocyanate leach solution. Also,the acidic effluent liquid 132 may be a more concentrated solutionformed by removal of water, such as by evaporation, from a lessconcentrated solution produced during the oxidative pretreatment 130.When mixed with the thiocyanate leach solution during the leach solutionconditioning 112, the acidic effluent liquid 132 preferably has a pH ofno larger than pH 3 and preferably no larger than ph 2.5. In onevariation, the acidic effluent liquid 132, when mixed with thethiocyanate leach solution during the leach solution conditioning 112,may have a pH of pH 2 or less or even pH 1.5 or less. In one variation,the acidic effluent liquid 132 has a pH in a range of from pH 0.1 to pH3, and preferably in a range of from pH 1 to pH 3, when added to thethiocyanate leach solution during the leach solution conditioning 112.

[0070] Use of the acidic effluent liquid 132 as a source of ferric ironmay be advantageous, for example, when a significant amount ofthiocyanate leach solution is being recycled from gold recoveryoperations for reuse to prepare the feed of thiocyanate leach solution106 (such as recycle of the effluent of the thiocyanate leach solution122 from the gold recovery operation 118 shown in FIG. 2). During thethiocyanate leach 104, a considerable quantity of ferric iron istypically reduced to ferrous iron, so that recycled thiocyanate leachsolution will typically be deficient in ferric iron relative to theconcentration of dissolved ferric iron in the feed of thiocyanate leachsolution 106. Nonlimiting examples of processes for potential use duringthe oxidative pretreatment 130 include bio-oxidation (such as bytreatment in a heap, tank or vat), pressure oxidation (with or withoutadditions such as of sulfuric acid, nitric acid or chlorine-containingreagents), nitric acid oxidation (such as by treatment in a heap, tankor vat) and chlorination (such as by treatment in a heap, tank or vatwith a hypochlorite reagent or another chlorine-containing oxidizingreagent). When the refractory mineral material feed 128 comprisesrefractory sulfide mineral material, preferred processing options foruse during the oxidative pretreatment 130 include bio-oxidation,pressure oxidation, and nitric acid oxidation. When the refractorymineral material feed 128 comprises refractory carbonaceous mineralmaterial, examples of processing options for use during the oxidativepretreatment include pressure oxidation and chlorination. When therefractory mineral material feed 130 comprises both refractory sulfidemineral material and refractory carbonaceous mineral material, oneexample of a processing option for the oxidative pretreatment 130includes pressure oxidation.

[0071] In one variation of the implementation shown in FIG. 3, theoxidative pretreatment 130 involves pressure oxidation, and especiallyfor treatment of a refractory sulfide mineral material feed. Referringnow to FIG. 4, a generalized process block diagram is shown for oneimplementation of such a variation. Reference numerals in FIG. 4 are thesame as used in FIGS. 1-3, except as noted. As shown in FIG. 4, therefractory mineral material feed 128 (which for exemplary purposes isdescribed as a refractory sulfide mineral material feed) is subjected topressure oxidation pretreatment 136. The mineral material 102, includingresidual solids from the pressure oxidation 136, is then subjected tothe thiocyanate leach 104 to dissolve gold from the residual solids. Inone embodiment, the acidic effluent liquid 132, including some acidicliquid produced during the pressure oxidation pretreatment 136, mayoptionally be supplied to the leach solution conditioning 112, toprovide a source of ferric iron, and for pH adjustment as desired. Whenused, such acidic effluent liquid 132 from the pressure oxidationpretreatment 136 preferably includes a concentration of dissolved ferriciron of larger than 0.05 M and more preferably larger than 0.1 M.

[0072] During the pressure oxidation pretreatment 136, at least aportion of sulfide sulfur in the refractory material feed 128 isoxidized to the sulfate form, destroying sulfide minerals to releasegold for recovery during the thiocyanate leach 104. The pressureoxidation pretreatment 136 is conducted in one or more reactors,typically referred to as autoclaves, and the acidic effluent liquid 132may be, for example, acidic aqueous liquid discharged from theautoclave(s).

[0073] Referring now to FIG. 5, a more specific variation of theimplementation of FIG. 4 is shown. Reference numerals in FIG. 5 are thesame as for FIGS. 1-4, except as noted. As shown in FIG. 5, the pressureoxidation pretreatment 136 includes the general steps of autoclaveprocessing 138 and solids conditioning 142. During the autoclaveprocessing 138, the refractory sulfide mineral material feed 128 ispressure oxidized in one or more autoclave(s) at elevated temperatureand pressure and with purified oxygen gas being introduced into theautoclave(s). As fed to the autoclave processing 138, the refractorymineral material feed 128 will preferably be in a finely groundparticulate form slurried with an aqueous liquid.

[0074] The conditions in the autoclave(s) during the autoclaveprocessing may be any conditions effective to produce the desiredpressure oxidation of the refractory sulfide mineral material feed. Theautoclave processing 138 is typically conducted at an elevatedtemperature, with the temperature in the autoclave(s) often reaching atleast 160° C., but the temperature in the autoclave(s) is often muchhigher. In some situations, the temperature in the autoclave(s) may beas high as 235° C., or even higher. The total pressure in theautoclave(s) during pressure oxidation processing will generally includethe vapor pressure exerted by noncondensible gases in the autoclave(s)(such as carbon dioxide and sulfur dioxide) and the overpressure ofoxygen gas supplied to the autoclave(s). By oxygen gas overpressure, itis meant the amount that the pressure of the oxygen gas fed to anautoclave exceeds the pressure exerted by the other components in theautoclave, such as water vapor and noncondensible gases. Even at lowprocessing temperatures, such as around 160° C., the total pressure maybe 85 psi (586 kPa) or more, while at higher temperatures, such asaround 225° C., the total pressure may be 485 psi (3,344 kPa) or more.Oxygen gas is fed to the autoclave(s) at an overpressure of typically atleast 10 psi (68.9 kPa) and frequently at least 25 psi (172.4 kPa). Whenoperating at high temperatures, the oxygen gas overpressure may be ashigh as 100 psi, (689 kPa), or even as high as 125 psi (862 kPa), orhigher. A more preferred operating range for pressure oxidation is atemperature of from 180° C. to 225° C., a total pressure of from 155 psi(1,069 kPa) to 460 psi (3,172 kPa) and oxygen gas overpressure of from25 psi (172 kPa) to 100 psi (689 kPa). The pressure oxidation occurringduring the autoclave processing 138 typically results in at least 80percent and preferably at least 90 percent of sulfide sulfur in therefractory mineral material feed 128 being oxidized, preferably to asulfate form. To effect the desired extent of pressure oxidation duringthe autoclave processing 138, a single autoclave may be used, which mayinclude multiple compartments arranged in series. In anotherimplementation, a plurality of autoclaves may be arranged in series.When using multiple compartments within an autoclave or multipleautoclaves in series, each of the different compartments or thedifferent autoclaves may independently be operated at differentconditions. Moreover, the autoclave processing may involve a singlepressure oxidation processing train or may involve multiple parallelpressure oxidation processing trains.

[0075] With continued reference to FIG. 5, oxidized slurry 140 from theautoclave processing 138 is processed through the solids conditioning142 to prepare the mineral material 102 to be supplied to thethiocyanate leach 104. The oxidized slurry 140 is a highly acidic slurrycontaining residual solids and acidic aqueous liquid, with the liquidcontaining typically 10 grams per liter or more of free sulfuric acidand also typically containing a significant concentration of dissolvedferric iron. In some instances, the free sulfuric acid may be at least20 grams per liter or even at least 50 grams per liter, although thefree sulfuric acid will also often be no larger than 100 grams perliter. The concentration of dissolved ferric iron will often be at least0.05 M (approximately 2.8 g/L) and preferably at least 0.1 M(approximately 5.58 g/L), although the concentrate of dissolved ferriciron will also most often be no larger than 0.2 M.

[0076] During the solids conditioning 142, any operation or combinationof operations may be implemented to prepare residual solids of theoxidized slurry 140 for use to prepare the mineral material 102 forprocessing in the thiocyanate leach 104. Such operations may include,for example, liquid-solid separation, dilution, partial neutralizationof solids, addition or reagents, washing solids and repulping solids. Inone variation, the solids conditioning 142 includes liquid-solidseparation to separate most or all of the acidic liquid in the autoclavedischarge from the solids, optionally followed by washing of the solidswith water to remove additional acidic liquid and increase the pH of thesolids to a higher, but still acidic, pH for the thiocyanate leach 104.If additional pH adjustment is desired, a neutralizing agent (such asfor example lime, limestone or a hydroxide) may be added. In onepreferred implementation, the mineral material 102, will include all orsome of the residual solids from the autoclave processing 138 in a denseslurry at an acidic pH, preferably at an acidic pH in a range of from pH1 to pH 3.

[0077] One feature shown in FIGS. 4 and 5 is the optional use of all ora portion of acidic liquid produced during the pressure oxidationpretreatment 136 for the optional supply of the acidic effluent liquid132 to the leach solution conditioning 112 for use to prepare the feedof the thiocyanate leach solution 106. With the implementations of FIGS.4 and 5, the acidic effluent liquid 132 would typically be liquidseparated from oxidized slurry discharged from pressure oxidationautoclave(s). The acidic effluent liquid 132 may be provided to theleach solution conditioning 112 with or without pH adjustment or otherintermediate processing. Referring to FIG. 5, in one variation theoxidized slurry 140 may be subjected during the solids conditioning 142to liquid-solid separation to separate acidic liquid from the solids,with at least a portion of the separated acidic liquid being optionallysupplied as the acidic effluent liquid 132 to the leach solutionconditioning. The solids may also be washed if desired. Wash water maybe combined with separated acidic liquid to prepare the acidic effluentliquid 132, or may be processed separately.

[0078] One variation for the processing shown in FIGS. 4 and 5 is to notseparately deliver acidic effluent liquid 132 directly to the leachsolution conditioning 112, and to feed the mineral material 102 to thethiocyanate leach 104 slurried with acidic autoclave discharge liquid,with or without dilution, pH adjustment or other treatment. For example,referring to FIG. 5, a portion of the acidic liquid may be separatedfrom the oxidized slurry 140 during the solids conditioning 142 and aportion of the acidic autoclave discharge liquid may be left with thesolids to be fed with the mineral material 102 to the thiocyanate leach104, providing a source of ferric iron for the thiocyanate leach 104. Inthis case, even though not fed separately to the thiocyanate leach 104,the acidic liquid with the mineral material 102 is considered part ofthe feed of the leach solution 106 and contributes to achieving desiredconditions in the thiocyanate leach with respect to dissolved ferriciron and dissolved thiocyanate, as previously described. As anotheralternative, the mineral material 102 may be fed to the thiocyanateleach 104 without the addition of the acidic effluent liquid 132 to theleach solution conditioning 112 and without acidic autoclave dischargeliquid being slurried with the mineral material 102.

[0079] Referring again to FIG. 3, when the oxidative pretreatment 130includes oxidation of the ore in a reactor (such as typically is thecase with pressure oxidation pretreatment and may be the case withbio-oxidation pretreatment), then the thiocyanate leaching 104 will alsotypically be performed in a reactor. By reactor, it is meant one or morefluid containment vessels that contain the materials during theparticular processing operation. For example, in the case of thepressure oxidation pretreatment 136 as shown in FIG. 5, the autoclaveprocessing 138 includes pressure oxidation performed in a reactor (oneor more pressure vessels generally referred to as autoclaves) and thethiocyanate leach 104 is performed in another reactor, such as forexample one or more stirred tanks, vats or other vessels.

[0080] In another variation of the implementation shown in FIG. 3, theoxidative pretreatment 132 involves bio-oxidation. This is one preferredoption for treating a refractory sulfide mineral material feed.Referring now to FIG. 6, a generalized process block diagram is shownfor one embodiment involving bio-oxidation as an oxidative pretreatmentstep. Reference numerals in FIG. 6 are the same as for FIG. 3, except asnoted. As shown in FIG. 6, the refractory mineral material feed 128(which for exemplary purposes is described as a refractory sulfidemineral material feed) is subjected to bio-oxidation pretreatment 146 tooxidize at least a portion of sulfide sulfur in the refractory mineralmaterial feed 128, thereby decomposing sulfide minerals and freeing goldfor recovery during the thiocyanate leach 104. The bio-oxidationpretreatment 146 may be performed in a reactor, such as for example oneor more stirred tank or other vessel, in which case the thiocyanateleach 104 will also preferably be performed in a reactor, such as forexample one or more stirred tank or other vessel. In a preferredvariation, the bio-oxidation pretreatment 146, and also the thiocyanateleach 104, are performed in a heap, due to especially advantageousprocessing in a heap with the present invention. The embodiment shown inFIG. 6 is therefore described primarily with reference to processing ina heap.

[0081] With continued reference to FIG. 6 in relation to a heapoperation, the bio-oxidation pretreatment 146 typically involvescirculating acidic bio-leach solution through a heap initiallycontaining the refractory mineral material feed 128. Circulation of theacidic bio-leach solution continues until oxidation of sulfide mineralshas progressed to a desired extent. After the heap has been sufficientlybio-oxidized, remaining solids provide the mineral material 102 for thethiocyanate leach 104. During the thiocyanate leach 104, the feed ofthiocyanate leach solution 106 is applied to the heap, such as forexample through a drip irrigation system, and the thiocyanate leachsolution percolates through the heap to dissolve gold in the form ofgold-thiocyanate complex. The pregnant thiocyanate leach solution 108 iscollected as it drains from the heap. The residual solids 110 remainingin the heap following the thiocyanate leach 104 are depleted in gold.

[0082] As shown in FIG. 6, acidic effluent liquid 132 produced duringthe bio-oxidation pretreatment 146 may optionally be provided to theleach solution conditioning 112 for use to prepare the feed ofthiocyanate leach solution 106. The acidic effluent liquid 132 is richin dissolved ferric iron and may be used to help adjust dissolved ferriciron levels in the leach solution and to adjust pH, in a manner aspreviously discussed. Because significant ferric iron is generatedduring the bio-oxidation pretreatment 146, the acidic effluent liquid132 will typically have a very high concentration of ferric iron. Theacidic effluent liquid 132 in the bio-oxidation implementation of FIG. 6will typically contain a concentration of dissolved ferric iron of atleast 0.1 M (approximately 5.6 g/L), more typically at least 0.3 M (16.7g/L) and preferably at least 0.4 M (approximately 22.3 g/L), althoughthe concentration of dissolved ferric iron will also often be no largerthan 0.8 M. Moreover, the acidic effluent liquid 132 is preferably at apH of from pH 1 to pH 3.

[0083] When processing in a heap environment, one enhancement for theoperation is to agglomerate mineral material feed prior to or when themineral material feed is deposited to initially form the heap. Forexample, with reference to FIG. 6, during agglomeration the refractorymineral material feed 128 in particulate form may be mixed with anacidic bacterial inoculate liquid. The agglomeration may be accomplishedby mixing particles of the mineral material with the bacterial inoculateliquid under conditions to promote agglomeration of the particles wettedwith the bacterial inoculate liquid into larger aggregate units. Forexample, the bacterial inoculate liquid could be sprayed onto theparticulate refractory mineral material 128 with the particulatematerial being processed through a rotating drum prior to or during thespraying to promote the agglomeration.

[0084] One implementation for performing such an agglomeration is shownin FIG. 7. As shown in FIG. 7, the refractory mineral material feed 128in particulate form is fed from a hopper 150 to a conveyor 152. Whiletraveling on the conveyor 152, the refractory mineral material feed 128is sprayed with a spray 154 of an acidic sulfate inoculate liquid from aspray apparatus 156. The refractory mineral material feed 128 is thentransferred through additional conveyors 158 and 160 and deposited toform a heap 162, which may then be subjected to bio-oxidation. The useof the multiple conveyors 152, 158 and 160 as shown in FIG. 7 assists tothoroughly mix particles of the refractory mineral material feed 128 andthe sprayed inoculate solution. Also, the action of the mineral materialspilling from one conveyor to another promotes agglomeration ofparticles of the refractory mineral feed 128 into larger aggregateunits, which improves properties, such as permeability, of the heap 162for subsequent bio-oxidation and thiocyanate leaching operations.

[0085] An important aspect of the implementation of the presentinvention as shown in FIG. 6 is that it is not necessary to remove theheap after the bio-oxidation pretreatment 146 and redeposit a new heapprior to the thiocyanate leach 104. Both the bio-oxidation pretreatment146 and the thiocyanate leach 104 are conducted at an acidic pH, andneutralization of solids in the heap is not required between thebio-oxidation 146 and the thiocyanate leach 104. The heap therefore mayremain substantially structurally undisturbed between completion of thebio-oxidation pretreatment 146 and commencement of the thiocyanate leach104, and it is not necessary to redeposit the solids in a new heap priorto performing the thiocyanate leach 104.

[0086] When operating a bio-oxidation/thiocyanate leach operation asgenerally described with reference to FIG. 6, the operation may at anygiven time involve a number of heaps in various stages of processing.For example, one or more heaps may be undergoing the bio-oxidationpretreatment 146 while one or more other heaps are simultaneouslyundergoing the thiocyanate leach 104. The heap or heaps undergoing thebio-oxidation pretreatment 146 operate at an acidic pH and produceacidic sulfate solutions having high concentrations of dissolved ferriciron. A typical pH for bio-leach solution is less than pH 2.5, and moretypically in a range of from pH 1.3 to pH 2.0. Any suitableiron-oxidizing acidophilic microorganism may be used for thebio-oxidation pretreatment 146. Examples of such microorganisms includethiobacillus ferrooxidans, leptospirillum ferrooxidans, sulfobocillusthermosulfidooxidans, metallospheara, sedula and Acidianus brierley.Additional information concerning bio-oxidation in general is providedin U.S. Pat. Nos. 5,246,486; 5,332,559; 5,834,294; 5,127,942 and5,244,493, the entire contents of each of which are incorporated hereinby reference as if set forth herein in full.

[0087] An important variation of the present invention involvingbio-oxidation followed by thiocyanate leaching is that the acidiceffluent liquid collected from a heap being subjected to bio-oxidationmay optionally be used to prepare a thiocyanate leach solution feed tobe applied to another heap being subjected to the thiocyanate leach.Reference is now made to FIG. 8 showing one implementation of thepresent invention in which bio-oxidation and thiocyanate leaching aresimultaneously performed on different heaps. Reference numerals are thesame as those used in FIGS. 2, 3 and 6, except as noted. In theembodiment shown in FIG. 8, a first heap 170 is undergoing thebio-oxidation pretreatment 146 while a second heap 172, which hasalready been pretreated by bio-oxidation, is simultaneously beingsubjected to the thiocyanate leach 104. In practice, these two heaps maybe at distant locations from each other or may be located adjacent toone another, and in the latter case there is preferably an impermeablebarrier placed between the heaps 170 and 172 to prevent fluidcommunication between the heaps 170 and 172.

[0088] As shown in FIG. 8, acidic feed of acidic bio-leach solution 174is applied to the first heap 170, such as for example by drip irrigationor another technique. Acidic effluent of the bio-leach liquid 175draining from the first heap 170 is collected and a major portion issent to treatment 176 for reuse to prepare the feed of the bio-leachsolution 174 that is applied to the first heap 170. During the treatment176, the pH may be adjusted, typically upward to a desired range for thebio-oxidation pretreatment 146, components such as iron and arsenic maybe precipitated, liquid may be added or removed as needed and otherreagents may be added as desired. Raising the pH and precipitating ironand arsenic may be accomplished, for example, by treating the solutionwith lime or some other neutralizing agent to partially neutralize thesolution and precipitate the unwanted components.

[0089] Simultaneously, with operation of the bio-oxidation pretreatment146 on the first heap 170, the feed of thiocyanate leach solution 106 isapplied to the second heap 172, such as for example by drip irrigationor another technique. The pregnant thiocyanate leach solution 108draining from the second heap 172 is collected and sent to the goldrecovery 118, where gold is removed from the pregnant thiocyanate leachsolution 108 and the purified gold product 120 is prepared. The barreneffluent of thiocyanate leach solution 122 from the gold recovery 118 issent to the leach solution conditioning 112 for use to prepare the feedof thiocyanate leach solution feed 106. A portion of the effluent ofbio-leach solution 175, which is rich in dissolved ferric iron, providesthe acidic effluent liquid 132 supplied to the leach solutionconditioning 112.

[0090] A significant enhancement available when combining bio-oxidationpretreatment with thiocyanate leaching with the present invention isthat the bio-oxidation pretreatment may be operated for only a shorttime prior to commencing the thiocyanate leach relative to bio-oxidationoperations performed prior to cyanide leaching. Also, the combinedduration of the bio-oxidation pretreatment and the thiocyanate leach maybe achieved in a short duration, relative to a combined bio-oxidationand cyanide leach operation. When operating a traditionalbio-oxidation/cyanide leach operation, it is necessary to oxidize thesulfide sulfur content in the feed of mineral material to such an extentthat no significant additional oxidation of sulfide sulfur occurs duringthe subsequent cyanide leaching operations. Acid produced as a result ofany such additional oxidation of sulfide sulfur would interfere withoperation of a cyanide leach, because the cyanide leach must beconducted at an alkaline pH. With the present invention, however, it isnot necessary to prevent further oxidation of sulfide sulfur during thethiocyanate leach operation, because the thiocyanate leach operation ispreformed at an acidic pH, and significant additional sulfide sulfuroxidation may be accommodated during the thiocyanate leach, so long asthe pH and other properties of the thiocyanate leach solution aremaintained within acceptable ranges.

[0091] During a traditional bio-oxidation/cyanide leach operation, thebio-oxidation pretreatment often lasts for 180 days or more. Thebio-oxidation time required for any particular ore material will dependupon a number of variables, including ore mineralogy and the coarsenessof the particle size of the treated material. A significant advantagewith the present invention is that it is often possible to reduce thebio-oxidation pretreatment time. As one example, for some mineralmaterials, the duration of the bio-oxidation pretreatment preceding athiocyanate leach with the present invention may be only half as long,or even shorter, relative to the duration that would be required toprecede a cyanide leach. In this example, a mineral material requiring180 days of bio-oxidation pretreatment prior to a conventional cyanideleach may in some cases require only 90 days or less of bio-oxidationpretreatment prior to the thiocyanate leach. One reason that a shorterduration may be sufficient with the present invention is that continuedoxidation of sulfide minerals, such as pyrite, during acidic thiocyanateleaching of gold does not create the same type of pH management problemsthat would be present during normal alkaline cyanide leaching. With thepresent invention, the bio-oxidation pretreatment may be discontinuedfor example when the sulfide sulfur oxidation is less than 30%, althoughthe total amount of sulfide sulfur oxidized during both thebio-oxidation and the thiocyanate leach may be larger than 30%, and mayoften be significantly larger than 30%, such as perhaps 50% or more.When operating with a shortened duration for the bio-oxidationpretreatment, as may be the case with the present invention, a ratio ofthe quantity of sulfide sulfur oxidized during the thiocyanate leach tothe quantity of sulfide sulfur oxidized during the bio-oxidationpretreatment may in one implementation be at least 1:10, in anotherimplementation at least 1:5, in yet another implementation be at least1:4 and in still another implementation be at least 1:3. One preferredrange for such a ratio is from 1:5 to 1:2.

[0092] In addition to processing refractory sulfide ores andconcentrates of such ores, the present invention is also useful forprocessing ores, and concentrates made from such ores, that include onlya moderate level of sulfide sulfur and which, therefore, are only mildlyrefractory to direct cyanide leaching for gold recovery. Such mildlyrefractory ores have a sulfide sulfur content of typically smaller than2 weight percent, often no greater than 1.5 weight percent, andsometimes even no greater than 1 weight percent. Such ores will,however, typically have a sulfide sulfur content of at least 0.5 weightpercent. As noted previously, these ores may be referred to astransition ores when significant gold is associated with oxide mineralsand with sulfide minerals.

[0093] The method of the present invention is particularly useful fortreating mildly refractory sulfide ores, because these ores may often beprocessed with the present invention without an oxidation pretreatmentstep and without the cumbersome and expensive pH modification requiredfor cyanide leaching operations. With this implementation of the presentinvention involving the treatment of mildly refractory mineralmaterials, it is preferred that at least 50 percent of the gold, andoften at least 70 percent of the gold, is cyanide leachable gold, asdetermined by laboratory testing described below. These mildlyrefractory mineral materials tend to produce significant quantities ofacid, which may significantly complicate and add to the expense ofrecovering the gold by direct cyanide leaching.

[0094] When processing mildly refractory sulfide ores, in oneimplementation the ore is subjected to an acid wash pretreatment priorto thiocyanate leaching. Referring now to FIG. 9, a generalized processblock diagram is shown for one such implementation of the presentinvention for treating a mildly refractory material. Reference numeralsare the same as in FIG. 1, except as noted. As shown in FIG. 9, a mildlyrefractory mineral material feed 180 in particulate form, such as forexample mildly refractory ore and/or a concentrate prepared from amildly refractory ore, is subjected to an acid wash 182, preferably withan acidic sulfate wash solution. During the acid wash 182, oxidation ofeasily oxidizable portions of the sulfide minerals is promoted, therebypromoting further release of gold. The acidic wash solution applied tocontact the mildly refractory mineral material 180 may be at any acidicpH, but is preferably at a pH of no higher than pH 4, more preferably ata pH of no higher than pH 3, often in a range of from pH 0.1 to pH 3,and more often in a range of from pH 1 to pH 3. The acid wash 182 mayinclude bio-oxidation pretreatment, but in one variation the acidicsulfate wash solution does not have added bacteria for promoting suchbio-oxidation pretreatment. The acid wash 182 may be continued for anyperiod of time, and it is preferably continued until oxidation of easilyoxidized sulfide sulfur in the mineral material feed 180 has proceededto a desired extent. Following the acid wash 182 remaining solids in theform of the mineral material 102 are subjected to the thiocyanate leach104.

[0095] In another implementation of the present invention, a method isprovided for treating a mineral material containing coppermineralization that includes appreciable soluble copper. As used herein,“soluble copper” refers to copper that would be susceptible to beingdissolved into the thiocyanate leach solution if present during anacidic thiocyanate leach according to the present invention, if notremoved prior to the acidic thiocyanate leach. Such soluble copper wouldalso typically be susceptible to being dissolved in cyanide leachsolutions were the mineral material instead subjected to cyanideleaching. For example, the mineral material may be a gold-bearing oxideore or concentrate that includes a nuisance quantity of soluble copper.Such nuisance quantities of copper may often be present in ores in arange of from a few hundred ppm soluble copper to a few thousand ppmsoluble copper, and possibly more. Such nuisance copper is frequently oflittle or no value and complicates gold recovery when using traditionalcyanide leaching to dissolve gold, because copper is easily dissolved bycyanide leach solutions and results in high cyanide consumption. Also,copper cyanide presents a disposal issue that may require specialprocessing. Examples of mineralizations that may contribute to thesoluble copper content in a mineral material include elemental copper,copper-containing oxide minerals, and secondary copper-containingsulfide minerals, such as for example may be the case with chalcocite(Cu₂S) or covellite (CuS) as secondary sulfide minerals.

[0096] Alternatively, the soluble copper may be present in high enoughquantities to be of economic value. The copper may represent the primaryvalue in the ore (the gold being a by product value) or the gold mayrepresent the primary value in the ore (the copper being a by productvalue).

[0097] In some instances when soluble copper represents the primaryvalue in the ore, it is not economic during conventional processing torecover by-product gold after copper recovery, and the gold represents alost economic value. This may be the case, for example, when copper isrecovered from lower-grade copper ores in acidic dump leachingoperations. In dump leaching operations, large piles of coarsecopper-containing rock are leached with an acidic sulfate solution todissolve copper that is easily solubilized. It is often not economic torecover gold with a cyanide leach following the acidic dump leach,because the cyanide leach has to be performed under alkaline conditions.

[0098] Referring now to FIG. 10, a generalized process block diagram isshown for one implementation of the present invention for treating ores,concentrates or other mineral materials that include an appreciablequantity of soluble copper. Reference numerals are the same as used inFIG. 1, except as noted. As shown in FIG. 10, a solublecopper-containing mineral material feed 186 is subjected to an acidiccopper leach 188. During the copper leach 188, the solublecopper-containing mineral material feed 186 is leached with an acidicleach solution, typically an acidic sulfate leach solution, to dissolvesoluble copper. Preferably, most or essentially all of the solublecopper is dissolved into the acidic leach solution during the copperleach 188. The soluble copper-containing mineral material feed 186 mayinclude, for example, a gold-bearing oxide ore, transition ore and/or aconcentrate that contains appreciable quantities of soluble copper. Asanother example, the soluble copper-containing mineral material feed 186may include a copper ore or concentrate that contains by-product gold.Feed of the acidic leach solution supplied to the copper leach 188 willhave an acidic pH, preferably will have a pH of no larger than pH 4,more preferably will have a pH of no larger than pH 3, typically willhave a pH in a range of from pH 0.1 to pH 3 and more typically will havea pH in a range of from pH 1 to pH 3. When it is determined that thesoluble copper has been sufficiently leached from the solublecopper-containing mineral material feed 186, then the copper leach 188is discontinued and remaining solids in the form of the mineral material102 are subjected to the thiocyanate leach 104 to dissolve gold. In apreferred variation, the copper leach 188 and the thiocyanate leach 104are conducted in a heap operation. An even more preferred variation isfor the copper leach 188 and the thiocyanate leach 104 to be conductedin the same heap, such that it is not necessary to disturb the structureof the heap between the copper leach 188 and the thiocyanate leach 104and it is not necessary to redeposit a new heap for the thiocyanateleach. Performing the copper leach 188 and the thiocyanate leach in thesame heap is possible because advantageously both operations are carriedout under acidic conditions.

[0099] The soluble copper content of the copper-containing mineralmaterial feed 186 may vary depending upon the situation. When thecopper-containing mineral material feed 186 is an ore containing only anuisance quantity of soluble copper, the soluble copper content willtypically be at least 200 ppm (0.02 weight percent), but is often atleast 500 ppm (0.05 weight percent) or even at least 1000 ppm (0.1weight percent) or more. When the copper-containing mineral materialfeed 186 is a concentrate made from such an ore, the soluble coppercontent of the concentrate may be significantly smaller, significantlylarger or about the same as the soluble copper content in the ore,depending upon the characteristics of the specific ore. When thecopper-containing mineral material feed 186 is an ore containingsufficient soluble copper for the soluble copper to be a valuablecomponent of the ore, the soluble copper content of the ore is often atleast 0.1 weight percent, may be at least 0.5 weight percent, or may beat least 1 weight percent or more. When the soluble copper representsthe primary metal value in the ore, the soluble copper content may be atleast 2 weight percent, or even at least 5 weight percent or more. Whenthe copper-containing mineral material feed 186 is a concentrate of suchan ore, the soluble copper content is preferably significantly largerthan the soluble copper content of the ore from which the concentratewas prepared. Even in the case of a concentrate, however, the solublecopper content will often be no larger than 20 weight percent.

[0100] When the soluble copper is only present in a nuisance quantity,then effluent acidic leach solution loaded with dissolved copper thatresults from the copper leach 188 may be subjected to water treatment toremove the dissolved copper and prepare the copper for disposal. Whenthe soluble copper represents a valuable product, the effluent leachsolution loaded with dissolved copper that results from the copper leach188 may be further processed for copper recovery. Such copper recoverymay involve, for example, iron cementation or solvent extraction. Onepreferred implementation for copper recovery involves solvent extractionof the dissolved copper from the acidic leach solution.

[0101] In another implementation, the present invention may involveprocessing a mineral material containing valuable precious metal valuesand valuable non-ferrous base metal values, wherein a significantportion of one or both of the precious metal values and the nonferrousbase metal values is contained within sulfide minerals that must bedecomposed to release the metal values for recovery. By non-ferrous basemetal it is meant a base metal other than iron. Examples of non-ferrousbase metals include copper, zinc, nickel, cobalt, indium and chromium.Particularly preferred for this implementation are mineral materialscontaining copper and/or zinc as nonferrous base metal values.

[0102] In this implementation, an acidic thiocyanate leach to dissolveprecious metals follows an acidic pretreatment operation during whichsulfide minerals are decomposed and non-ferrous base metal values areleached. The acidic pretreatment combines oxidative pretreatment todecompose sulfide minerals to release precious metal and/or non-ferrousbase metal for recovery and leaching of the non-ferrous base metal. Onespecific implementation is shown in FIG. 24. Reference numerals in FIG.24 are the same as for FIG. 2, except as noted.

[0103] As shown in FIG. 24, a mineral material feed 220 is subjected toa pretreatment leach 222. During the pretreatment leach 222, sulfideminerals in the mineral material feed 220 are decomposed and non-ferrousbase metal is dissolved to form an acidic pregnant leach solution 224containing the dissolved non-ferrous base metal. The mineral material102, including residual solids from the pretreatment 222, is thenprocessed in the thiocyanate leach 104 to dissolve gold. The pregnantleach solution 224 is then subjected to base metal recovery 226. Duringthe base metal recovery 226, the non-ferrous base metal is removed fromthe pregnant leach solution 224 to prepare purified product(s) includingnon-ferrous base metal. For example, when processing a copper-gold oreor concentrate, during the base metal recovery 226 the copper may beremoved from the pregnant leach solution 224 by solvent extraction intoan organic extractant phase, and the extracted copper may then bestripped from the organic extractant phase into an aqueous stripsolution and a copper-containing product prepared from the aqueous stripsolution, such as for example by electrowinning or precipitation. Asanother example, when processing a zinc-gold ore or concentrate, duringthe base metal recovery 226 the zinc may be removed from the pregnantleach solution 224 by solvent extraction of the zinc or by selectiveprecipitation of the zinc as a hydroxide. As yet a further example, whenprocessing a copper-zinc-gold ore or concentrate, the base metalrecovery 226 may include, for example, solvent extraction to selectivelyremove copper from the pregnant leach solution 224 followed by selectiveprecipitation of zinc as a hydroxide.

[0104] In another aspect, the present invention involves removal ofprecious metal, and particularly gold, from a pregnant thiocyanate leachsolution, such as may result from thiocyanate leaching of a preciousmetal-containing mineral material. Referring again to FIG. 2, during thegold recovery 118, gold-thiocyanate complex may be removed from thepregnant thiocyanate leach solution 108 by adsorption onto activatedcarbon, in a manner similar to removing gold-cyanide complex fromcyanide leach solutions. The rate of adsorption of gold-thiocyanatecomplex onto activated carbon is, however, very slow. Also, activatedcarbon promotes reduction of dissolved ferric iron in the thiocyanateleach solution to ferrous iron, which is generally not desirable. As analternative, gold-thiocyanate complex may be removed from pregnantthiocyanate leach solution by ion exchange resin. Ion exchange resin,however, is expensive and use of ion exchange resin may require complexprocessing to selectively elute gold-thiocyanate complex. With thepresent invention, neither adsorption of gold-thiocyanate complex ontoactivated carbon nor the recovery of gold-thiocyanate complex onexchange resin is preferred as a technique for remaining gold from apregnant thiocyanate leach solution, although each may be used, ifdesired.

[0105] One preferred technique with the invention for removing preciousmetal, and particularly gold, from a pregnant thiocyanate leach solutionis solvent extraction. Solvent extraction is effective for removing goldfrom the acidic thiocyanate leach solution and good phase separation ofthe organic and aqueous phases may be attained. Also, solvent extractiondoes not tend to promote reduction of ferric iron in the thiocyanateleach solution, as does the use of activated carbon. During the solventextraction, the acidic aqueous pregnant thiocyanate leach solution iscontacted with an organic phase that includes an extractant for removalof gold-thiocyanate complex from the pregnant thiocyanate leachsolution. Some examples of extractants for use in the organic phaseinclude phosphorous-containing extractants, such as phosphate,phosphonate, phosphinate and phosphine oxide extractants. Phosphates areparticularly preferred as phosphorous-containing extractants for usewith the invention. Other examples of extractants include amineextractants, which may comprise primary, secondary or tertiary amines.The use of amine extractants is particularly preferred for use with thepresent invention to remove gold-thiocyanate complex from acidicpregnant thiocyanate leach solutions by solvent extraction. Followingextraction into the organic phase, gold-thiocyanate complex may then beremoved from the loaded organic phase by stripping into an aqueous stripsolution, such as for example an alkaline aqueous strip solution. Not tobe bound by specific mechanism, but to aid in understanding of theinvention, the extraction of gold-thiocyanate complex using amineextractants is believed to proceed generally as follows:

[0106] 1) An amine acid salt is formed,

SCN⁻+RNH₂+H⁺=(RNH₃ ⁺.SCN⁻)

[0107] 2) Amine-acid and gold-thiocyanate complex form an ion-pair,

(RNH₃ ⁺.SCN⁻)+Au (SCN)₂ ⁻═(RNH₃.Au(SCN)₂ ⁻)+SCN⁻

[0108] One implementation of the invention involving solvent extractionto recover gold is shown in the generalized process block diagram ofFIG. 11. Reference numbers are the same as in FIG. 2, except as noted.As shown in FIG. 11, the pregnant thiocyanate leach solution 108 issubjected to the gold recovery 118, which comprises the generalprocessing steps of solvent extraction 192, stripping 194 andelectrowinning 196.

[0109] During the solvent extraction 192, the aqueous pregnantthiocyanate leach solution 108 is contacted with an organic liquid phasecontaining an extractant for gold-thiocyanate complex. Gold-thiocyanatecomplex is transferred from the pregnant thiocyanate leach solution 108into the organic liquid phase. The organic liquid phase loaded with goldis separated from the thiocyanate leach solution, and the barreneffluent of the thiocyanate leach solution 122 is supplied to the leachsolution conditioning 112 for use to prepare the feed of the thiocyanateleach solution 106.

[0110] During the stripping 194, the loaded organic liquid phase iscontacted with an aqueous strip solution to transfer gold from theorganic liquid phase into the strip solution. The strip solution may be,for example an alkaline aqueous solution. The strip solution loaded withgold is then subjected to the electrowinning 196 to prepare the purifiedgold product 120.

[0111] In another preferred embodiment of the present invention,precious metal, and particularly gold, is removed from an acidicpregnant thiocyanate leach solution by transferring precious metal inthe solution from the precious metal-thiocyanate complex to a newcomplex with a second complexing agent, with the new complex being morereadily removable from the pregnant thiocyanate leach solution. In apreferred variation, the second complexing agent is cyanide and the newcomplex is precious metal-cyanide complex, and particularly gold-cyanidecomplex. When a small amount of cyanide is added to a pregnantthiocyanate leach solution containing gold-thiocyanate complex, goldquickly transfers in the solution from gold-thiocyanate complex togold-cyanide complex. The pregnant thiocyanate leach solution may thenbe processed to remove gold-cyanide complex. For example, gold-cyanidecomplex may be removed from the pregnant thiocyanate leach solution byany technique used to remove gold-cyanide complex from cyanide leachsolutions in conventional cyanide leaching operations.

[0112] In a preferred variation of this implementation, after transferof gold to the gold-cyanide complex, the gold-cyanide complex is thenremoved from the pregnant leach solution by using an adsorbent materialto adsorb gold-cyanide complex. The adsorbent material may be, forexample carbon (preferably activated carbon) or ion exchange resin.Preferably the absorbent material comprises a carbonaceous adsorbent,such as granules of activated carbon. Once loaded with gold-cyanidecomplex, the absorbent material may be processed to recover gold in thesame manner as with conventional cyanide leach operations. For example,gold-cyanide complex loaded onto activated carbon granules may bestripped from the granules into a hot caustic strip solution or someother strip solution, and the gold may then be recovered in a purifiedproduct, such as by electrowinning.

[0113] One such preferred variation of the invention is shown in thegeneralized process block diagram of FIG. 12. Reference numerals are thesame as in FIG. 2, except as noted. As shown in FIG. 12, after thethiocyanate leach 104, the pregnant thiocyanate leach solution 108 issent to the gold recovery 118, which in this implementation comprisesthe general processing steps of complex transfer 200, complex absorption202, complex stripping 204 and electrowinning 206. During the complextransfer 200, dissolved cyanide is introduced into the pregnantthiocyanate leach solution 108 and gold dissolved in the pregnantthiocyanate leach solution 108 is transferred from gold-thiocyanatecomplex to gold-cyanide complex. During the complex adsorption 202,gold-cyanide complex is loaded onto an adsorbent material, such as forexample activated carbon granules or ion exchange resin. The barrenthiocyanate leach solution 122 from which the gold-cyanide complex hasbeen removed is supplied to the leach solution conditioning 112 for useto prepare the feed of thiocyanate leach solution 106.

[0114] Because gold-cyanide complex is thermodynamically favored overgold-thiocyanate complex, during the complex transfer 200 the golddissociates from thiocyanate complexing agent to form a complex withcyanide complexing agent. During the complex transfer 200, preferably atleast 80 percent, more preferably at least 90 percent, even morepreferably at least 95 percent and most preferably at least 98 percentof gold dissolved in the pregnant thiocyanate leach solution 108 istransferred from gold-thiocyanate complex to gold-cyanide complex.

[0115] The complex transfer 200 and the complex adsorption 202 may beperformed separately or together. For example, the dissolved cyanide maybe introduced into the pregnant thiocyanate leach solution 108 prior toadding the adsorbent material, to permit most of the gold to transfer togold-cyanide complex prior to contacting the pregnant thiocyanate leachsolution 108 with the adsorbent material. In a preferred alternative,however the adsorbent material is added prior to or at about the sametime as introduction of the dissolved cyanide, so that the complextransfer 200 and the complex adsorption 202 proceed concurrently. Duringthe complex stripping 204, gold-cyanide complex is removed from theloaded adsorbent material by stripping the gold-cyanide complex into anaqueous strip solution. Stripped adsorbent material may be recycled forreuse during the complex adsorption 202 to be loaded with additionalgold-cyanide complex. During the electrowinning 206, gold is removedfrom the loaded strip solution by electrowinning to prepare the purifiedgold product 120. During the complex transfer 200 and the complexadsorption 202, the thiocyanate leach solution is preferably at anacidic pH, and more preferably at a pH in a range of from pH 1 to pH 3.Preferably the thiocyanate leach solution is maintained at such anacidic pH throughout the thiocyanate leach 104, the complex transfer 200and the complex adsorption 202.

[0116] With the implementation of the invention involving transfer ofprecious metal from precious metal-thiocyanate complex to preciousmetal-cyanide complex, removal of precious metal-cyanide complex byloading onto an adsorbent material, such as shown in FIG. 12, ispreferred. Even though not preferred, however, other techniques may beused instead to remove precious metal from the pregnant leach solutionfollowing transfer of precious metal to the precious metal-cyanidecomplex. For example, gold-cyanide complex may be removed from thepregnant leach solution by solvent extraction or gold may be removedfrom the pregnant leach solution by cementation.

[0117] The transfer of gold from gold-thiocyanate complex togold-cyanide complex during the complex transfer 200 may advantageouslybe accomplished in a preferred implementation of the invention byaddition to the pregnant thiocyanate leach solution 108 of only a smallquantity of dissolved cyanide. A stoichiometric quantity of cyaniderequired for complete complexation with the gold to from gold-cyanidecomplex is two moles of the cyanide group CN per mole of gold, assumingall gold is solubilized as the aurocyanide ion Au(CN)₂ ⁻. The disclosedcyanide may be introduced into the pregnant thiocyanate leach solution108 in any suitable form, such as for example in the form of sodium orpotassium cyanide. Moreover, the cyanide may be introduced into thepregnant thiocyanate leach solution 108 in any convenient manner, suchas for example, by dissolving a cyanide reagent (e.g. sodium orpotassium cyanide) into the pregnant thiocyanate solution 108, or(preferably) by adding to the pregnant thiocyanate leach solution 108 asmall quantity of a pre-prepared, concentrated cyanide solution. Also,the quantity of the cyanide added to the pregnant thiocyanate leachsolution 108 will typically be at a molar ratio of the added cyanide toprecious metal (and preferably of the added cyanide to gold) of nolarger 20:1 (ten times a stoichiometric quantity), preferably no largerthan 10:1 (5 times a stoichiometric quantity), more preferably no largerthan 5:1 (2.5 times a stoichiometric quantity) and even more preferablyno larger than 4:1 (two times a stoichiometric quantity). The quantityof added cyanide will typically be at a molar ratio of cyanide toprecious metal (and preferably of the added cyanide to gold) of at least2:1 (a stoichiometric quantity). Moreover, the cyanide will typically beadded to the pregnant thiocyanate leach solution 108 in a quantity thatis small in comparison to the amount of dissolved thiocyanate in thepregnant thiocyanate leach solution 108. The quantity of added cyanidewill typically be at a molar ratio of cyanide to dissolved thiocyanateof no larger than 1:2, preferably no larger than 1:4, more preferably nolarger than 1:5, even more preferably no larger than 1:7 and still morepreferably no larger than 1:10.

[0118] After introduction of the dissolved cyanide into the pregnantthiocyanate leach solution 108, some or all of residual dissolvedcyanide may be converted to thiocyanate in the thiocyanate leachsolution, preferably after removal of precious metal-cyanide complexfrom the thiocyanate leach solution. The conversion of residual cyanidein the thiocyanate leach solution to the thiocyanate provides a sourceof new thiocyanate lixiviant to partially or completely compensate forthiocyanate losses that occur during gold leaching and/or gold recoveryoperations. Also, although the use of activated carbon as an absorbentmaterial tends to promote reduction of ferric iron in the thiocyanateleach solution to ferrous iron, kinetics of adsorption of gold-cyanidecomplex onto activated carbon are much faster than for adsorption ofgold-thiocyanate complex, resulting in the use of shorter contact timeswith activated carbon granules and/or lower concentrations of activatedcarbon granules, thereby significantly lessening the potential problemassociated with such reduction of ferric iron.

[0119] In another implementation of the invention, a portion or all ofthe dissolved thiocyanate in a feed of the thiocyanate leach solution isprovided by conversion of dissolved cyanide to dissolved thiocyanate.For example, a thiocyanate leach solution may be initially prepared bydissolving cyanide in an acidic sulfate solution and then converting thedissolved cyanide in the solution to dissolved thiocyanate. Theconversion may take place either before or during thiocyanate leachingof a mineral material. For example, leaching of gold from a heap couldbe commenced with a leach solution initially including dissolved cyanidethat is converted to dissolved thiocyanate as the acidic leachprogresses. Moreover, make-up thiocyanate may be supplied to an existingthiocyanate leach solution to compensate for thiocyanate losses overtime by the addition of small quantities of cyanide, which then convertin solution to thiocyanate. This may be done in conjunction with goldrecovery operations, as discussed above, or separately. This techniqueis useful for providing dissolved thiocyanate in a thiocyanate leachsolution using cyanide reagents, such as sodium or potassium cyanide,rather than by dissolving potentially more expensive thiocyanatereagents, such as for example sodium thiocyanate, potassium thiocyanateor ammonium thiocyanate.

[0120] Referring now to FIG. 13, a generalized process block diagram isshown for one variation of this implementation. As shown in FIG. 13,during cyanide addition 212, cyanide is added to an aqueous liquid sothat the aqueous liquid, after the cyanide addition 212, containsdissolved cyanide. The cyanide may be added, for example, as apre-prepared concentrated cyanide solution added to the aqueous liquidor in a solid salt form that is dissolved into the aqueous liquid. Thecyanide may be added in any desired form, such as for example in theform of sodium cyanide or potassium cyanide. After being dissolved inthe aqueous liquid, at least a portion of the dissolved cyanide, andpreferably substantially all of the dissolved cyanide, is converted todissolved thiocyanate during the conversion 214, to prepare the feed ofthiocyanate leach solution 106. During the conversion 214, typically atleast 80 mole percent, preferably at least 90 mole percent and morepreferably substantially all of the dissolved cyanide converts todissolved thiocyanate. Preferably, both the cyanide addition 212 and theconversion 214 are performed at an acidic pH, more preferably in a rangeof from pH 1 to pH 3. The aqueous liquid may or may not already containdissolved thiocyanate prior to the cyanide addition 212. As an example,the cyanide addition 212 and conversion 214 of FIG. 13 may be performedas part of the leach solution conditioning 112 of any of the embodimentsshown in any of FIGS. 1-6, 8-12 and 24. As another example, the cyanideaddition 212 and the conversion 214 of FIG. 13 may involve initialpreparation of a thiocyanate solution prior to commencement ofthiocyanate leaching operations.

[0121] In one preferred variation of the implementation of FIG. 13,rapid conversion of dissolved cyanide to dissolved thiocyanate ispromoted during the conversion 214 by introducing appropriate reagent(s)into the aqueous liquid during the conversion 214. Preferred reagentsfor converting cyanide to thiocyanate include sulfide and hydrosulfidematerials. Examples of possible reagents include sodium sulfide, sodiumhydrosulfide, potassium sulfide, potassium hydrosulfide, ammoniumsulfide, ammonium hydrosulfide and hydrogen sulfide. Other examples ofpossible reagents include sulfide minerals, such as for examplepyrrhotite.

[0122] As another example, some or all of the conversion of cyanide tothiocyanate could occur during thiocyanate leaching, such as due tocontact with sulfide minerals, such as pyrrhotite, present in themineral material being leached. For example, referring again to FIG. 12,some or all of cyanide introduced into the thiocyanate leach solutionfor the complex transfer 200 could subsequently be converted tothiocyanate during the thiocyanate leach 104 by contact with sulfideminerals, such as for example pyrrhotite, in the mineral material 102.

EXAMPLES

[0123] The following examples are provided to further aid understandingof the present invention and not to limit the scope of the invention.

Example 1 Column Leach of Mildly Refracting Ore

[0124] Tests are performed on a mildly refractory sulfide ore from theLone Tree mine located in Nevada, U.S.A. XRD/XRF semiquantitativeanalysis indicates the ore to be composed of about 86% quartz, 6%kaolin, 3% pyrite, 2% alunite, 1% gypsum, 1% jarosite and 1% barite.Representation assay information for the ore samples is shown in Table3. The ore does not exhibit preg-robbing tendencies. TABLE 3Representative Ore Sample Assay Au S-Total⁽²⁾ S-Sulfide⁽³⁾ Fe As g/t⁽¹⁾weight % weight % weight % ppm 2.26 1.643 0.951 1.551 655.9

[0125] Two sets of leach tests are performed, one using a cyanide leachsolution and the other using a thiocyanate leach solution. All tests areperformed on minus 10 mesh (1.68 mm) ore samples placed in 2 inch (50.8mm) diameter columns for leaching.

[0126] For the cyanide leach tests, 200 grams of ore sample, alone ormixed with lime for neutralization, are loaded into the column andleached with an aqueous sodium cyanide (NaCN) leach solution to a weightratio of final solution to ore of 2.68 (i.e., each 200 gram sample istreated with 536 grams of leach solution). The leach solution initiallycontains 125 ppm NaCN and is at pH 9.8-10. Results of the cyanide leachtests are summarized in Table 4. As shown in Table 4, the maximum goldextraction with cyanide is achieved in the test using the largest amountof lime, and therefore operating at the most alkaline pH. Gold recoveryis particularly low when no lime is used. FIG. 14 shows a plot of goldextraction vs. cumulative solution-to-ore weight ratio for each of thetests, further demonstrating the beneficial effect of operation of thecyanide leach at an alkaline pH. TABLE 4 Cyanide Leach Test Results TestCN-1 CN-2 CN-3 CN-4 Lime Addition (lb/st)⁽¹⁾ 15 10 5 0 Final Solution toOre 2.68 2.68 2.68 2.68 Ratio⁽²⁾ Final Pregnant Leach 9.1 7.0 5.2 4.0Solution pH Gold Extraction (%)⁽³⁾ 56.67 51.52 44.24 30.61 GoldExtraction (%)⁽⁴⁾ 58.55 53.62 46.67 32.65

[0127] For the thiocyanate leach tests, an aqueous thiocyanate leachsolution is prepared using either ammonium thiocyanate (NH₄SCN) orpotassium thiocyanate (KSCN) reagent dissolved in water. Ferric sulfate(Fe₂(SO₄)₃) is added to the leach solution to obtain a desired quantityof dissolved ferric iron and a desired oxidation-reduction potential.The thiocyanate leach solution is initially at about pH 2. For eachtest, 800 grams ore sample is loaded into the column, and the sample isleached with the thiocyanate leach solution. The leach solution istested using different concentrations of dissolved thiocyanate anddissolved ferric iron, and leaching is conducted to various finalsolution-to-ore weight ratios. Results of the thiocyanate leaching testsare summarized in Table 5. As shown in Table 5, gold extractionsachieved in the thiocyanate leach tests are generally as high as orhigher than gold extractions achieved during the cyanide leach testsinvolving significant lime addition. This is particularly noteworthybecause each of the thiocyanate leach tests is conducted at an acidicpH. TABLE 5 Thiocyanate Leach Test Results Test SCN-1 SCN-2 SCN-3 SCN-4SCN-5 SCN-6 SCN Concentration (M)⁽¹⁾ 0.05 0.05 0.02 0.02 0.01 0.01 Fe³⁺Concentration (M)⁽²⁾ 0.2 0.1 0.2 0.1 0.2 0.1 Fe³⁺/SCN Molar Ratio⁽³⁾ 4 210 5 20 10 Weight Ratio of Final Solution 4.125 4.112 3.066 2.546 4.3004.505 to Ore Gold Extraction (%)⁽⁴⁾ 61.13 59.78 57.26 59.38 59.59 53.48Gold Extraction (%)⁽⁵⁾ 61.31 60.14 56.61 59.90 54.42 51.83

Example 2 Bottle Roll Leach Following Bio-Oxidation Pretreatment ofRefractory Sulfide Ore

[0128] Tests are preformed on samples of a refractory sulfide gold orefrom the Lone Tree mine. A 65.8 kg sample of ore crushed to 100% passing2 inches (50.8 mm) is subjected to bio-oxidation pretreatment in acolumn having an inside diameter of 11 inches (279.4 mm), to simulatebio-oxidation in a heap. Prior to placing the ore in the column, 1920 mLof a mixed culture of acidophilic iron-oxidizing bacteria, containingAcidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, is mixedwith the ore. During bio-oxidation a nutrient solution containing 0.4g/L (NH₄)₂ SO₄, 0.4 g/L MgSO₄ 7H₂O and 0.04 g/L K₂HPO₄, is continuouslyrecirculated through the ore in the column at a flow rate of about 6.5mL/min. The column is continuously aerated from the base at an air flowrate of 28.3 L/h. The bio-oxidation pretreatment is continued for 258days at room temperature (approximately 20-22° C.). Upon termination ofthe bio-oxidation pretreatment, the ore is removed from the column.During the bio-oxidation pretreatment, about 35% of the sulfide sulfuris oxidized. Representative assay information for the bio-oxidized oreis shown in Table 6. TABLE 6 Representative Bio-Oxidized Ore SampleAssay Au Total Sulfur Sulfide Sulfur Fe Cu As g/t⁽¹⁾ Weight % Weight %Weight % ppm ppm 2.81 3.14 2.72 2.63 30.46 1853

[0129] Following completion of the bio-oxidation, the bio-oxidized oreis washed to remove soluble iron, dried and then crushed and ground to aP₈₀ size of about 200 mesh (74 microns) for gold extraction testing. AP₈₀ size means that 80 weight percent of the sample passes a screenopening of the noted size. For gold extraction testing, a sample of thecrushed and ground bio-oxidized ore is placed in a 1 gallon (3.78 L)bottle along with freshly prepared thiocyanate leach solution at a pulpdensity of 20 weight percent solids. Several leach tests are performedusing thiocyanate leach solutions of approximately pH 2 made withvarying concentrations of potassium thiocyanate (KSCN) and ferricsulfate (Fe₂(SO₄)₃) dissolved in deionized water.

[0130] During the thiocyanate leach tests, the bottles are open to theair and solution samples are taken at time intervals. The leaching lastsfor a total of about 6 hours. Solution potential, pH values, andthiocyanate concentrations are measured. Gold is analyzed by atomicabsorption spectrophotometry (AAS). To overcome matrix effects, all AAScalibration standards are diluted in solutions representative of thethiocyanate leach solutions used in the particular leach tests. For sometests, gold concentration is also determined by solvent extraction withdi-isobutyl ketone (containing 1% Aliquat 336) and AAS analysis.Thiocyanate concentration is determined by Volhard titration, whichdetermines the total SCN concentration. Total iron concentration isdetermined by AAS and ferrous ion concentration is determined bytitration with potassium permanganate (KMnO₄) or potassium dichromate(K₂Cr₂O₇) in the presence of sulfuric acid after complete precipitationof the thiocyanate ion. The solids residue from each thiocyanate leachtest is washed thoroughly with water and dried prior to analysis of arepresentative sample for gold using fire assay followed by digestionand AAS.

[0131] Table 7 summarizes the molar concentrations of thiocyanate,ferric iron and cyanide in the prepared leach solutions. Results aresummarized in Table 8. Table 8 reports solution oxidation-reductionpotential (designated as E). E, as used herein, refers to solutionoxidation-reduction potential expressed in millivolts as measured usinga platinum (Pt) working electrode relative to a silver/silver chloride(Ag/AgCl) reference electrode. Eh (oxidation-reduction potentialrelative to a standard hydrogen electrode) may be determined from thereported E values by correction relative to a standard hydrogenelectrode. TABLE 7 Prepared Leach Solutions Ratio Test [CN] M [SCN] M[F_(e) ³⁺] M [F_(e) ³⁺]:[SCN] SCN-7 — 0.01 0.1 10 SCN-8 — 0.02 0.1 5SCN-9 — 0.05 0.1 2 SCN-10 — 0.01 0.2 20 SCN-11 — 0.02 0.2 10 SCN-12 —0.05 0.2 4 SCN-13 — 0.01 0.3 30 SCN-14 — 0.02 0.3 15 SCN-15 — 0.05 0.3 6

[0132] TABLE 8 Leach Results Pregnant Leach Pregnant Leach Solution EGold Test Solution pH mV Extraction % SCN-7 1.98 481 39.81 SCN-8 1.95476 41.10 SCN-9 1.91 462 50.05 SCN-10 1.88 498 47.49 SCN-11 1.87 49350.14 SCN-12 1.85 480 56.55 SCN-13 1.75 509 45.10 SCN-14 1.74 504 48.60SCN-15 1.74 492 55.00

[0133]FIG. 15 shows plots of gold extraction vs. initial thiocyanateconcentration in the thiocyanate leach solution for the tests involving0.1 M ferric iron, 0.2 M ferric iron and 0.3 M ferric iron. As seen inFIG. 15, the tests with 0.2 M and 0.3 M ferric iron consistently showedsignificantly higher gold recoveries than the corresponding test withonly 0.1 M ferric iron, for the same concentration of thiocyanate.

Example 3 Bottle Roll Leach Following Bio-Oxidation Pretreatment ofRefractory Sulfide Ore

[0134] Bottle roll leach tests are performed as described in Example 2using the biooxidized ore of Example 2. Tests are performed usingfreshly prepared thiocyanate leach solutions containing dissolvedthiocyanate and ferric iron as shown in Table 9. Leaching continues fora total of 24 hours for each test. For each test, leach solution samplesare obtained and analyzed at 2 hours, 4 hours, 6 hours 12 hours and 24hours. For comparison, cyanide leachable gold for this ore is determinedto be 69%. By “cyanide leachable gold”, it is meant gold extracted froma sample of the mineral material (e.g., the ore or the concentrate) by astandardized test procedure as follows:

[0135] A 5 gram sample of the mineral material pulverized to a size ofminus 200 mesh is placed into a test tube along with 10 ml of a solutioncontaining 0.3 weight percent sodium cyanide and 0.3 weight percentsodium hydroxide in ionized water. The test tube is rotated for one hourat room temperature. The solution and remaining solids are thenseparated by centrifuge, and gold content in the solution is determinedby atomic absorption analysis. Cyanide leachable gold is then determinedas the percent gold extraction into the solution based on solution goldcontent (determined from the atomic absorption analysis) relative to theoriginal gold content in the mineral material (determined by fire assayanalysis). TABLE 9 Prepared Leach Solutions [SCN] [F_(e) ³⁺] Molar RatioTest M (g/L) M (g/L) [F_(e) ³⁺]:[SCN] SCN-16  0.01 (0.58) 0.2 (11.1) 20SCN-17  0.02 (1.16) 0.2 (11.1) 10 SCN-18 0.05 (2.9) 0.2 (11.1) 4 SCN-19 0.1 (5.8) 0.2 (11.1) 2 SCN-20 0.05 (2.9) 0.1 (5.58) 2 SCN-21 0.05 (2.9)0.3 (16.7) 6

[0136] From the cyanide leach test, it is determined that about 69% ofthe gold in the bio-oxidized sample is cyanide leachable. The highestgold extraction for the thiocyanate leach tests is about 65%. Table 10summarizes total gold extraction during 24 hours of leaching for each ofthe leach tests. TABLE 10 Gold Extraction Test Gold Extraction (%)SCN-16 50.0 SCN-17 60.49 SCN-18 64.15 SCN-19 63.05 SCN-20 62.93 SCN-2165.49

[0137] Table 11 summarizes data for several tests concerning propertiesof the thiocyanate leach solution determined from solution samplesobtained periodically during the tests. Data summarized in Table 11includes changes in thiocyanate leach solution pH and E, reduction offerric to ferrous iron in the thiocyanate leach solution and degradationof thiocyanate. FIG. 16 shows a plot of total gold extraction vs.thiocyanate concentration through 6 hours of leaching for tests SCN-16,17, 18 and 19, which each use the same concentration of dissolved ferriciron (0.2 M) and different concentrations of dissolved thiocyanate.Notably, gold extractions are higher for test SCN-18 than SCN-19, eventhough SCN-19 uses a lower initial concentration of dissolvedthiocyanate in the thiocyanate leach solution. FIG. 17 shows a plot ofgold extraction vs. ferric iron concentration for tests SCN-18, 20, and21, which each use the same concentration of dissolved thiocyanate (0.05M) and different concentrations of dissolved ferric iron. As seen inFIG. 17, gold extraction for these tests increases with increasinginitial concentrations of ferric iron. TABLE 11 Thiocyanate SolutionAnalysis Test/Property 2 hours 4 hours 6 hours 12 hours 24 hours TestSCN-17 pH 1.69 1.69 1.71 1.64 1.58 E, mV 509 508 505 499 492 Fe²⁺ Assay,M 0.038 0.039 0.045 0.050 0.059 Reduction Fe³⁺ to 19. 19.5 22.5 25 29.5Fe²⁺, % SCN Assay, g/L 0.99 1.05 — 1.04 1.05 Test SCN-18 pH 1.73 1.731.71 1.66 1.60 E, mV 494 490 488 482 476 Fe²⁺ Assay, M 0.046 0.051 0.0580.063 0.070 Reduction Fe³⁺ to 23 26 29 31.5 35 Fe²⁺, % SCN Assay, g/L2.70 1.75 2.16 2.22 2.66 Test SCN-19 pH 1.72 1.72 1.74 1.67 1.65 E, mV479 478 475 471 465 Fe²⁺ Assay, M 0.056 0.062 0.066 0.071 0.080Reduction Fe³⁺ to 28 31 33 35.5 40 Fe²⁺, % SCN Assay, g/L 2.85 2.41 1.641.70 1.43 Test SCN-20 pH 1.94 1.94 1.88 1.83 1.74 E, mV 476 490 488 482476 Fe²⁺ Assay, M 0.040 0.043 0.043 0.052 0.056 Reduction Fe³⁺ to 40 4343 52 56 Fe²⁺, % SCN Assay, g/L 2.74 2.75 2.08 1.15 2.08 Test SCN-21 pH1.58 1.59 1.61 1.56 1.52 E, mV 506 502 500 494 489 Fe²⁺ Assay, M 0.0490.058 0.061 0.067 0.078 Reduction Fe³⁺ to 16.3 19.3 20.3 22.3 26 Fe²⁺, %SCN Assay, g/L 2.64 1.79 2.66 2.67 2.39

Example 4 Column Leach Following Bio-Oxidation Pretreatment ofRefractory Sulfide Ore

[0138] A refractory sulfide gold ore from the Lone Tree mine, of a typegenerally as described in Example 2, is biooxidized. Representativeassay information for the bio-oxidized ore is shown in Table 12. Aportion of the bio-oxidized ore is air dried and crushed to a size ofminus 10 mesh (1.68 mm) for gold extraction testing. TABLE 12Representative Bio-oxidized Ore Sample Assay Au Total Sulfur SulfideSulfur Fe (g/t)⁽¹⁾ (Weight %) (Weight %) (Weight %) 2.13 1.67 1.43 1.82

[0139] Gold extraction testing is performed by loading a sample of thebio-oxidized ore into a column having an internal diameter of 2 inches(50.8 mm). A freshly prepared acidic thiocyanate leach solution at aboutpH 2 is then passed through the bio-oxidized ore sample in the column,simulating a heap leach. A sample of pregnant thiocyanate leach solutionis periodically analyzed for gold extraction and other properties. Table13 summarizes concentrations of thiocyanate and ferric iron and themolar ratio of ferric iron to thiocyanate in the freshly preparedthiocyanate leach solutions for the tests. TABLE 13 Prepared ThiocyanateSolutions Molar Ratio Test [SCN] M [F_(e) ³⁺] M [F_(e) ³⁺]:[SCN] SCN-220.05 0.2 4 SCN-23 0.05 0.1 2 SCN-24 0.02 0.2 10 SCN-25 0.02 0.1 5 SCN-260.02 0.04 2 SCN-27 0.01 0.1 10 SCN-28 0.01 0.2 20

[0140]FIG. 18 shows a bar plot of gold extraction into the thiocyanateleach solution for each of the tests and a line plot of thiocyanateconsumption (pounds of SCN consumed per short ton of bio-oxidized oresample tested) for each of the tests.

Example 5 Column Leach Following Bio-Oxidation Pretreatment ofRefractory Sulfide Ore

[0141] A portion of the bio-oxidized ore of Example 4 is air dried andcrushed to a size of minus 32.8 mm for gold extraction testing. Aftercrushing, a 13.6 kg sample of the bio-oxidized ore is loaded into eachof three columns. Each column has an inside diameter of 4 inches (101.6mm). The bio-oxidized ore sample in each column is leached with either acyanide leach solution or a thiocyanite leach solution to extract gold.For the cyanide leach test, the bio-oxidized ore sample is agglomeratedwith lime at 6 kg per tonne of ore sample prior to being loaded into thecolumn. The amount of lime addition is determined based onneutralization tests performed on the same bio-oxidized ore. Thiocyanateleach solutions are prepared at a pH of about pH 2 with ferric sulfateand potassium thiocyanate dissolved in deionized water. The cyanideleach solution is prepared at a pH between 10.5 and 11 with 0.25 g/Lsodium cyanide. Properties of the prepared leach solutions aresummarized in Table 14. TABLE 14 Prepared Leach Solutions [NaCN] [SCN][F_(e) ³⁺+] Ratio Solution Solution Test g/L M M [F_(e) ³⁺+]:SCN] pHE⁽¹⁾ CN-5 0.25 — — — SCN-29 — 0.01 0.1 10 1.98 585 SCN-30 — 0.02 0.1  51.99 570

[0142] For each test, leach solution is applied at a rate of about 9.8L/hr-m² to the top of the biooxidized ore samples in the columns from areservoir of leach solution having an initial volume of 1900 mL. Thetest continues for 17 days.

[0143] For the cyanide leach test, gold is recovered from the pregnantcyanide leach solution by contacting the pregnant solution with a columncontaining activated carbon granules. Following recovery of the gold,additional sodium cyanide is added to the barren cyanide leach solutionto obtain 0.25 g/L sodium cyanide concentration and the barren cyanideleach solution is then recycled to the column for additional leaching.

[0144] For the thiocyanate leach tests, on each of days 1 through 10 andon days 13 and 16, the pregnant leach solution is removed and analyzed,and a fresh 1900 mL batch of leach solution is provided. On day 17, thepregnant leach solution is removed and analyzed. Also on day 17, thecolumn is rinsed with a rinse solution of acidified deionized water(acidified to pH 2 with sulfuric acid) and the rinse solution isanalyzed for gold content.

[0145]FIG. 19 shows a plot of gold extraction into the leach solutionvs. time for each of tests CN-5, SCN-29 and SCN-30 through 13 days ofleaching. FIG. 20 shows a plot of potassium thiocyanate consumption orsodium cyanide consumption (pounds of potassium thiocyanate or sodiumcyanide consumed per short ton of bio-oxidized ore sample treated) vs.time for each of tests CN-5, SCN-29 and SCN-30. Tables 15 and 16summarize results for tests SCN-29 and SCN-30. TABLE 15 Test SCN-29Results % Cumulative Au Pregnant Leach Solution Extraction ElapsedSolution/Ore From Solution Time Ratio [Au] [SCN] EhE⁽¹⁾ Assay Based on(days (Cumulative) (ppm) (ppm) [Fe²⁺] pH (mV) Head Analysis 1 0.126 3.55435.0 3228.1 1.32 489 21.28 2 0.259 2.37 536.5 1105.8 1.63 510 36.37 30.402 0.61 507.5 826.6 1.67 515 40.52 4 0.537 0.55 478.5 837.8 1.61 51744.06 5 0.676 0.45 464.0 893.6 1.59 524 47.04 6 0.824 0.33 493.0 826.61.59 521 49.37 7 0.955 0.17 464.0 804.2 1.60 518 50.44 8 1.094 0.17478.5 837.8 1.47 512 51.57 9 1.215 0.16 493.0 781.9 1.39 507 52.48 101.355 0.10 522.0 776.3 1.52 505 53.15 13 1.493 0.14 522.0 1212.0 1.37488 54.06 16 1.633 0.10 512.3 1340.0 1.39 482 54.73 17 1.769 0.01 145.0533.0 1.55 473 54.79

[0146] TABLE 16 Test SCN-30 Results % Cumulative Au Extraction PregnantLeach Solution From Elapsed Solution/Ore Solution Assay Time Ratio [Au][SCN] EhE⁽¹⁾ Based on Head (days (Cumulative) (ppm) (ppm) [Fe²⁺] pH (mV)Analysis 1 0.125 4.54 884.5 3083 1.32 487 27.10 2 0.263 1.99 1000.5 11621.61 501 40.23 3 0.397 0.81 1131.0 983 1.61 505 45.39 4 0.525 0.631102.0 1184 1.57 507 49.23 5 0.664 0.41 1000.5 1016 1.59 515 51.95 60.811 0.31 1000.5 961 1.51 511 54.12 7 0.945 0.14 1015.0 916 1.54 50955.01 8 1.084 0.11 971.5 949 1.53 506 55.75 9 1.205 0.16 1000.5 983 1.38501 56.65 10 1.346 0.10 1024.9 894 1.52 498 57.32 13 1.484 0.15 976.31251 1.37 482 58.31 16 1.625 0.11 995.7 1447 1.39 476 59.05 17 1.761138.0 503 1.54 470 59.05

EXAMPLE 6 Solvent Extraction Removal of Gold from Thiocyanate Solution

[0147] Acidic synthetic thiocyanate solutions are prepared by dissolvingpotassium thiocyanate, ferric sulfate and gold in deionized water.Properties of the prepared thiocyanate solutions are summarized in Table17. Two different organic liquid phases are prepared including Armeen™312 (Akzo Nobel) extractant. Armeen™ 312 is a tertiary amine(trilaurylamine) extractant. The first organic phase (O-1) is a solutionof 0.05 M Armeen™ 312 in kerosene. The second organic phase (O-2) is amixture of 0.9 part by volume of O-1 with 0.1 part by volume decanol(0.045 M Armeen™ 312). TABLE 17 Prepared Thiocyanate Solutions PregnantThiocyanate [Au] [SCN] [Fe]⁽¹⁾ E⁽²⁾ Solution ppm ppm ppm pH mV A-1 36.5957 4488 1.72 530 A-2 10.2 996 5074 1.74 549 A-3 4.79 1121 5105 1.76 556A-4 1.95 1083 5058 1.75 556

[0148] For each test, approximately 250 mL of pregnant aqueousthiocyanate solution is placed in a separatory funnel along with theorganic phase at a volumetric ratio of organic phase to aqueous phase ofeither 1:1 or 1:2. The separatory funnel is shaken on a mechanical wristshaker for approximately 10 minutes. The organic and aqueous phases areallowed to separate, and the aqueous raffinate and loaded organic phasesare then removed from the separatory funnel for analysis. Results aresummarized in Table 18. As seen in Table 18, gold recovery from theaqueous phase into the organic phase is high for all tests. Also,separation of the aqueous and organic phases following the solventextraction is excellent for all tests. TABLE 18 Solvent Extraction TestResults Gold Loading Aqueous Raffinate in Aqueous Organic O:A⁽¹⁾ [Au][Fe] [SCN] Gold Organic Test Solution Phase Ratio ppm ppm ppm pHRecovery % ppm SX-1 A-1 O-1 1:1 0.04 4829 116 1.84 99.89 36.46 SX-2 A-2O-1 1:1 0.01 4993 136 1.85 99.90 10.19 SX-3 A-3 O-1 1:1 0.02 4927 971.87 99.58 4.77 SX-4 A-4 O-1 1:1 0 5174 77 1.88 100.00 1.95 SX-5 A-1 O-11:2 0.11 4679 213 1.83 99.70 73.88 SX-6 A-2 O-1 1:2 0.04 4777 213 1.8399.61 20.23 SX-7 A-3 O-1 1:2 0.05 4916 174 1.82 98.96 9.62 SX-8 A-4 O-11:2 0.01 5064 213 1.82 99.49 3.94 SX-9 A-1 O-2 1:1 0.05 4679 174 1.9399.86 36.45 SX-10 A-2 O-2 1:1 0.02 4777 193 1.95 99.80 10.18 SX-11 A-3O-2 1:1 0.01 4916 174 1.95 99.79 4.78 SX-12 A-4 O-2 1:1 0.03 5064 1741.95 98.46 1.92 SX-13 A-1 O-2 1:2 0.18 4490 348 1.78 99.51 73.74 SX-14A-2 O-2 1:2 0.05 4430 348 1.81 99.51 20.61 SX-15 A-3 O-2 1:2 0.06 4797387 1.83 98.75 9.60 SX-16 A-4 O-2 1:2 0.01 4622 715 1.82 99.49 3.94

Example 7 Solvent Extraction Removal of Gold from Thiocyanate Solution

[0149] Concentrated solutions of different amine extractants areobtained and organic liquid phases including the different extractantsare prepared by diluting 0.2 part by volume of the concentrated solutionas received with 0.8 part by volume of xylene. An aqueous pregnantthiocyanate solution is prepared by column leaching a sample ofrefractory sulfide gold ore that has been pretreated by bio-oxidation.The pregnant thiocyanate leach solution from the column leach contains2.06 ppm dissolved gold, 899 ppm dissolved thiocyanate, and 6450 ppmtotal dissolved iron (with 290.4 ppm of the dissolved iron being ferrousiron), a pH of 1.5 and an E of 489 mV. The extractants tested areAlamine™ 336 (tertiary amine, tri-C₈-C₁₀-alkylamine, from Cognis),Amberlite™ LA-2 (secondary amine, lauryl-tert-alkylamine, from Cognis),Armeen™ (primary amine, dodecylamine, from Akzo Nobel), and Armeen™ 312(tertiary amine, trilaurylamine, from Akzo Nobel).

[0150] For each test, approximately 250 mL of pregnant aqueousthiocyanate leach solution is placed in a separatory funnel along withan approximately equal volume of the organic phase (0:A ratio=1:1). Theseparatory funnel is shaken on a mechanical wrist shaker forapproximately 10 minutes. The organic and aqueous phases are allowed toseparate, and the aqueous raffinate and loaded organic phases areremoved from the separatory funnel for analysis. Results are summarizedin Table 19. The best phase separation is obtained for test SX-21, butphase separation is good for all tests. TABLE 19 Solvent Extraction TestResults Organic Aqueous Raffinate Phase [Au] [Fe]⁽¹⁾ Gold TestExtractant PH ppm ppm Recovery % SX-17 Alamine 308 1.64 0.04 4384 98.06SX-18 Alamine 336 1.49 0.02 5046 99.03 SX-19 Amberlite 1.48 0 2994100.00 LA-2 SX-20 Armeen 1.05 0.25 5250 87.86 SX-21 Armeen 312 1.04 0.016279 99.51

Example 8 Solvent Extraction Removal of Gold From Pregnant ThiocyanateSolution

[0151] A synthetic aqueous thiocyanate solution is prepared in deionizedwater containing 2.21 ppm dissolved gold, 905 ppm (0.15 M) dissolvedthiocyanate, and 6470 ppm (0.11 M) total dissolved iron, and having a pHof 1.84 and an E of 493.3 mV. Tributylphosphate (“TBP”) is used as theorganic extractant phase at various volumetric ratios of TBP to thepregnant thiocyanate solution. For each test a total volume of thecombined TBP and pregnant thiocyanate solution of about 500 mL is placedin a separatory funnel and shaken on a mechanical wrist shaker for 10minutes, after which the organic and aqueous phases are allowed toseparate. The organic and aqueous phases are removed from the separatoryfunnel for analysis. Results are summarized in Table 20, including goldrecovery into the organic phase from the aqueous phase. Phase separationis good for all tests. TABLE 20 Solvent Extraction Test Results AqueousRaffinate Au Ferric Iron Gold Test O:A Ratio⁽¹⁾ (ppm) (ppm) Recovery %SX-22 1:1 0 6210 100.00 SX-23 1:2 0 6800 100.00 SX-24 1:5 0 6030 100.00SX-25  1:10 0 6290 100.00 SX-26  1:20 0.60 6040 98.64%

Example 9 Ion Exchange Removal of Gold from Thiocyanate Solution

[0152] A sample of Purolite™ 600, (Purolite Company), a gel-type, stronganionic ion exchange resin is obtained and divided into two portions.Water is removed from one portion by vacuum filtration and the dryweight of the resin is determined to provide information concerning themoisture content of the wet resin. The wet resin contains about 65.72weight percent resin and about 34.28 weight percent water. For eachtest, 480 mL of a pregnant thiocyanate solution is added to a 1 L flaskalong with 1 g of the Purolite™ 600 resin (containing about 0.6572 g ionexchange resin), and the contents of the flask are mixed by a magneticstirrer. Thiocyanate leach solution samples of 20 mL each are removedfrom the flask at the end of 1 hour, 3 hours and 7 hours and analyzed.Tests are performed using different levels of gold loading in thepregnant thiocyanate solutions containing either approximately 0.02 Mthiocyanate or approximately 0.05 M thiocyanate. Gold loading on theresin (based on dry resin weight) is determined at the end of 7 hours.

[0153] Test results are summarized in Table 21. FIG. 21 shows plots ofgold loading on the resin (dry resin basis) relative to goldconcentration in the thiocyanate solutions following adsorption of thegold by the resin. A separate plot is shown in FIG. 21 for those testsusing about 0.02 M thiocyanate and those tests using about 0.05 Mthiocyanate. FIG. 22 shows plots of the concentration of gold in thethiocyanate solution relative to time for test IX-2 (0.02 M thiocyanate)and test IX-5 (0.05 M thiocyanate). FIG. 23 shows plots of gold recoveryfrom the thiocyanate solution relative to time for tests IX-2 (0.02 Mthiocyanate) and IX-5 (0.05 M thiocyanate). As seen in Table 20 andFIGS. 21-23, ion exchange resin performance is better with lowerconcentrations of thiocyanate in the pregnant thiocyanate solution.TABLE 21 Ion Exchange Test Results Test Time/Properties IX-1 IX-2 IX-3IX-4 IX-5 IX-6 Time = 0 (Initial Solution) [Au], ppm 2.695 6.808 28.3002.569 6.938 27.450 [Fe], ppm⁽¹⁾ 4602 4633 4524 4630 4607 4540 [SCN], ppm1126.3 1140.2 1069.7 2664.5 2843.4 2692.1 pH 1.84 1.84 1.84 1.93 1.931.90 Time = 1 hr [Au], ppm 1.108 2.638 10.746 1.319 3.214 10.894 [Fe],ppm⁽¹⁾ 4342 4462 4673 4450 4474 4872 [SCN], ppm 905.2 905.9 917.8 2528.92605.3 2408.7 pH 1.93 1.93 1.90 1.98 1.99 1.90 Time = 3 hr [Au], ppm0.542 1.137 4.238 0.934 2.282 7.955 [Fe], ppm⁽¹⁾ 4553 4871 4631 47904477 4299 [SCN], ppm 859.7 862.6 588.2 2394.4 2583.2 2376.7 pH 1.94 1.941.91 2.03 2.01 1.99 Time = 7 hr [Au], ppm 0.267 0.591 2.691 0.567 1.2826.663 [Fe], ppm⁽¹⁾ 4342 4340 4331 4291 4017 4295 [SCN], ppm 860.3 789.9849 2431.7 2422.7 2531.1 pH 1.93 1.92 1.89 2.01 1.98 1.95 Resinloading⁽²⁾ 1773.34 4540.72 18,704.08 1462.20 4129.52 15,182.23

Example 10 Cyanide/Carbon Removal of Gold from Thiocyanate Solution

[0154] Four different aqueous thiocyanate solutions are prepared indeionized water. For each test, approximately 250 mL of preparedthiocyanate solution is placed in a 500 mL flask along with activatedcarbon granules at a concentration of about 20 grams of the activatedcarbon granules per liter of pregnant thiocyanate solution. Sodiumcyanide (NaCN) is added to the flask in sufficient quantity to provide amolar ratio of sodium cyanide to gold of 10. The contents of the flaskare agitated on a mechanical shaker for 2 hours, and then the contentsof the flask are removed and analyzed.

[0155] Properties of the prepared thiocyanate solution for each test andthe results of each test are summarized in Table 22. Gold recovery fromthe thiocyanate solution is good for all tests, but is somewhat betterfrom the thiocyanate solutions prepared with lower concentrations ofthiocyanate. TABLE 22 Cyanide/Carbon Adsorption Tests Final ThiocyanateInitial Prepared Thiocyanate Solution Solution NaCN [Au] E [Au] EAddition Au Test. ppm [SCN] M [Fe³⁺] M PH mV ppm PH mV ppm Recovery %C-1 5.09 0.02 0.1 1.8 587 0.13 1.61 471 12.7 97.45 C-2 5.09 0.05 0.1 1.8587 0.14 1.65 455 12.7 97.25 C-3 2.01 0.02 0.1 1.8 587 0.06 1.56 469 4.797.01 C-4 2.01 0.05 0.1 1.8 587 0.07 1.62 454 4.7 96.52

Example 11 Cyanide/Activated Carbon Removal of Gold from ThiocyanateSolution

[0156] A thiocyanate solution is prepared in deionized water containing5.25 ppm gold, 0.02 M thiocyanate, 0.1 M ferric iron and 0.0016 Mferrous iron and having a pH of 1.67 and an E of 560 mV. For each test,approximately 250 mL of the prepared thiocyanate solution is placed in a500 mL beaker along with activated carbon granules at a concentration of20 grams of the activated carbon granules per liter of the pregnantthiocyanate solution. Sodium cyanide (NaCN) is added to the flask insufficient quantity to provide a molar ratio of sodium cyanide to goldof 10. The contents of the flask are agitated on a mechanical shaker fora different length of time for each test. Following the agitation, thethiocyanate solution is analyzed to evaluate the kinetics of goldrecovery and reduction of ferric iron to ferrous iron.

[0157] Results are summarized in Table 23. As seen in Table 23, goldrecovery kinetics are fast, and reduction of ferric iron to ferrous ironis not excessive. TABLE 23 Cyanide/Carbon Adsorption Tests Elapsed FinalThiocyanate Solution Ferric Iron Au Time Au E Reduction Recovery TestHours ppm Fe²⁺ M pH mV % % C-5 0.5 1.41 0.010 1.62 492 9.60 73.14 C-6 10.30 0.006 1.51 428 6.40 94.29 C-7 2 0.05 0.006 1.40 480 6.40 99.05 C-84 0.06 0.008 1.26 461 8.40 98.86

Example 12 Cyanide/Activated Carbon Removal of Gold from ThiocyanateSolution

[0158] Tests are performed as described in Example 11, except thatdifferent molar ratios of sodium cyanide to gold are used and each testis run for two hours, after which the contents of the flask areanalyzed. Results are summarized in Table 24. Gold recovery is good evenusing a lower mole ratio of cyanide to gold. TABLE 24 Cyanide/CarbonAdsorption Tests Mole Ratio Final Thiocyanate Solution Ferric Iron Au[NaCN]: [Au] [Fe^(2+]) E Reduction Recovery Test [Au] ppm M PH mV % %C-9  5:1 0.07 0.008 1.37 479 8.01 98.67 C-10 10:1 0.05 0.006 1.40 4806.41 99.05 C-11 20:1 0.09 0.009 1.37 479 8.81 98.29

Example 13 Cyanide/Activated Carbon Removal of Gold From ThiocyanateSolution

[0159] Tests are performed as described in Example 11, except thatdifferent concentrations of activated carbon granules are used and eachtest is run for two hours, after which the contents of the flask areanalyzed. Results are summarized in Table 25. TABLE 25 Cyanide/CarbonAdsorption Tests Activated Final Thiocyanate Solution Ferric Iron AuCarbon [Au] [Fe^(2+]) Reduction Recovery Test g/L ppm M PH E mV % % C-125 0.08 0.006 1.39 501 6.41 98.48 C-13 10 0.05 0.008 1.37 489 7.61 99.05C-14 20 0.05 0.006 1.40 480 6.41 99.05

Example 14 Oxidation of Ferrous Iron to Ferric Iron in ThiocyanateSolution

[0160] A thiocyanate solution is prepared by dissolving ferrous sulfate(FeSO₄) and potassium thiocyanate in deionized water and adding sulfuricacid to adjust the pH to approximately pH 2. The thiocyanate solutioncontains 0.67 gram per liter (0.0120 M) dissolved ferrous iron and 1.96grams per liter (0.0337 M) dissolved thiocyanate. For each test, 200 mLof thiocyanate solution is placed in a 500 mL flask and potassiumpersulfate (K₂S₂O₈) is then added to the flask. The mixture in the flaskis agitated on a vibrating shaker for 10 minutes, and a 20 mL sample ofsolution is then collected from the flask and analyzed. The remainingsolution is allowed to sit in the flask for about 47 hours and 50minutes (total time 48 hours including time shaking) and is thenanalyzed. Results are summarized in Table 26. TABLE 26 Oxidation ofFerrous Iron in Thiosulfate Solution K₂S₂O₈ 10 minutes 48 HoursOxidation of Addition [Fe²⁺] [SCN] E [Fe²⁺] SCN E Fe²⁺ to Fe³⁺ % Test gMR⁽¹⁾ g/L g/L mV g/L g/L mV 10 mins 48 hrs OX-1 0.024 0.037 0.58 1.96388 0.58 1.96 375 13.4 13.4 OX-2 0.048 0.074 0.52 1.96 410 0.53 1.95 40522.4 20.9 OX-3 0.072 0.11 0.48 1.96 426 0.47 1.96 423 28.4 29.8 OX-40.096 0.15 0.43 1.96 437 0.43 1.97 435 35.8 35.8 OX-5 0.192 0.30 0.261.96 466 0.27 1.90 466 61.2 59.7 OX-6 0.363 0.56 0.19 1.94 487 ..20 1.95482 71.6 70.1 OX-7 0.484 0.75 0.08 1.84 512 0.19 1.91 503 88.1 85.1 OX-80.968 1.49 0.01 1.64 541 0.04 1.55 522 98.5 94.0

[0161] The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to only the form or forms specifically disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedhereinabove are further intended to explain best modes known forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart. Although the description of the invention has included descriptionof one or more possible implementations and certain variations andmodifications, other variations and modifications are within the scopeof the invention, e.g., as may be within the skill and knowledge ofthose in the art after understanding the present disclosure. It isintended to obtain rights which include alternative embodiments to theextent permitted, including alternate, interchangeable and/or equivalentstructures, functions, ranges or steps to those claimed, whether or notsuch alternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter. Furthermore, any featuredescribed or claimed with respect to any disclosed implementation may becombined in any combination with one or more of any other features ofany other implementation or implementations, to the extent that thefeatures are necessarily not technically compatible, and all suchcombinations are within the scope of the present invention.

[0162] The terms “comprise”, “include”, “have” and “contain”, andvariations of those terms, as may be used in relation to the presence ofa feature, are intended to indicate only that a particular feature ispresent, and are not intended to limit the presence of other features.The phrase “at least a portion” of a method means some or all of thematerial, and preferably a majority of the material.

What is claimed is:
 1. A method for processing precious metal-containingmineral material to recover precious metal from the mineral materialusing a circulating thiocyanate leach solution, the method comprising:thiocyanate leaching the mineral material with the thiocyanate leachsolution, the thiocyanate leach solution comprising dissolvedthiocyanate and dissolved ferric iron and during the thiocyanateleaching at least a portion of the precious metal from the mineralmaterial is dissolved into the thiocyanate leach solution in the form ofprecious metal-thiocyanate complex; supplying feed of the thiocyanateleach solution to the thiocyanate leaching, the feed of the thiocyanateleach solution comprising a pH in a range of from pH 1 to pH 3 andcomprising a molar ratio of the dissolved ferric iron to the dissolvedthiocyanate of at least 2, and during the thiocyanate leaching at leasta portion of the dissolved ferric iron in the feed of the thiocyanateleach solution is reduced to dissolved ferrous iron; recovering at leasta portion of the precious metal from the thiocyanate leach solution,after the recovering barren effluent of the thiocyanate leach solutioncomprises a concentration of the dissolved ferric iron that is smallerthan a concentration of the dissolved ferric iron in the feed of thethiocyanate leach solution; conditioning the thiocyanate leach solutionto prepare the feed of the thiocyanate leach solution, at least aportion of the barren effluent of the thiocyanate leach solution beingsupplied to the conditioning for use to prepare the feed of thethiocyanate leach solution, and the conditioning comprising increasing aconcentration of the dissolved ferric iron in the thiocyanate leachsolution relative to the concentration of the dissolved ferric iron inthe barren effluent of the thiocyanate leach solution supplied to theconditioning.
 2. The method of claim 1, wherein a concentration of thedissolved thiocyanate in the feed of the thiocyanate leach solution isno larger than 0.03 mole per liter.
 3. The method of claim 2, whereinthe concentration of the dissolved thiocyanate in the feed of thethiocyanate leach solution is in a range of from 0.0001 mole per literto 0.03 mole per liter.
 4. The method of claim 2, wherein theconcentration of the dissolved thiocyanate in the feed of thethiocyanate leach solution is in a range of from 0.001 mole per liter to0.02 mole per liter.
 5. The method of claim 1, wherein the concentrationof the dissolved ferric iron in the feed of the thiocyanate leachsolution is at least 0.05 mole per liter.
 6. The method of claim 1,wherein the concentration of the dissolved ferric iron in the feed ofthe thiocyanate leach solution is at least 0.1 mole per liter.
 7. Themethod of claim 1, wherein the molar ratio of the dissolved ferric ironto the dissolved thiocyanate in the feed of the thiocyanate leachsolution is at least
 4. 8. The method of claim 1, wherein aconcentration of the dissolved thiocyanate in the feed of thethiocyanate leach solution is in a range of from 0.0001 mole per literto 0.03 mole per liter, the concentration of the dissolved ferric ironin the feed of the thiocyanate leach solution is at least 0.05 mole perliter, and the molar ratio of the dissolved ferric iron to the dissolvedthiocyanate in the feed of the thiocyanate leach solution is at least 7.9. The method of claim 1, wherein the precious metal comprises gold andat least a majority of the gold in the mineral material is leached intothe thiocyanate leach solution during the thiocyanate leaching.
 10. Themethod of claim 1, wherein the recovering comprises solvent extractionof the precious metal from the thiocyanate leach solution after thethiocyanate leaching, the solvent extraction comprising contacting thethiocyanate leach solution with an organic extractant phase andtransferring at least a portion of the dissolved precious metal from thethiocyanate leach solution into the organic extractant phase.
 11. Themethod of claim 10, wherein the organic extractant phase comprises aphosphorous-containing extractant.
 12. The method of claim 10, whereinthe organic extractant phase comprises an amine extractant.
 13. Themethod of claim 1, wherein: the recovering comprises contacting thethiocyanate leach solution with ion exchange resin after the thiocyanateleaching; and during the contacting at least a portion of the preciousmetal is removed from the thiocyanate leach solution and loaded onto theion exchange resin.
 14. The method of claim 1, wherein the recoveringcomprises introducing dissolved cyanide into the thiocyanate leachsolution and transferring at least a portion of the precious metal inthe thiocyanate leach solution from the precious metal-thiocyanatecomplex to precious metal-cyanide complex.
 15. The method of claim 14,wherein the recovering comprises removing at least a portion of theprecious metal-cyanide complex from the thiocyanate leach solution. 16.The method of claim 15, wherein the removing comprises loading at leasta portion of the precious metal-cyanide complex onto an adsorbentmaterial.
 17. The method of claim 16, wherein the adsorbent materialcomprises activated carbon.
 18. The method of claim 16, wherein theadsorbent material comprises ion exchange resin.
 19. The method of claim14, wherein the introducing comprises adding the cyanide to thethiocyanate leach solution at a molar ratio of the cyanide to preciousmetal in the thiocyanate leach solution that is no larger than
 20. 20.The method of claim 19, wherein: the precious metal in the thiocyanateleach solution comprises gold; and the introducing comprises adding thecyanide to the thiocyanate leach solution at a molar ratio of thecyanide thiocyanate leach solution to the gold in the thiocyanate leachsolution that is no larger than
 20. 21. The method of claim 20, whereinthe molar ratio of the cyanide to the gold in the thiocyanate leachsolution is in a range of from 2 to
 20. 22. The method of claim 1,wherein the thiocyanate leach solution is maintained at an acidic pHduring the thiocyanate leaching, the recovering and the conditioning.23. The method of claim 1, wherein the thiocyanate leach solution ismaintained at a pH in a range of from pH 1 to pH 3 during thethiocyanate leaching, the recovering and the conditioning.
 24. Themethod of claim 1, wherein the conditioning comprises introducingdissolved cyanide into the thiocyanate leach solution and converting atleast a portion of the dissolved cyanide to thiocyanate prior tosupplying the feed of the thiocyanate leach solution to the thiocyanateleaching.
 25. The method of claim 1, wherein the conditioning comprisesintroducing dissolved cyanide into the thiocyanate leach solution andconverting substantially all of the dissolved cyanide to thiocyanateprior to supplying the feed of the thiocyanate leach solution to thethiocyanate leaching.
 26. The method of claim 1, wherein the thiocyanateleaching comprises: applying the feed of the thiocyanate leach solutionto a heap comprising the mineral material; and percolating thethiocyanate leach solution through the heap.
 27. The method of claim 26,comprising, prior to the thiocyanate leaching, preparing the mineralmaterial, the preparing comprising: forming the heap initiallycomprising particulate feed material, the particulate feed materialcomprising sulfide minerals; and bio-oxidizing the feed material in theheap to decompose at least a portion of the sulfide minerals.
 28. Themethod of claim 1, wherein the conditioning comprises adding acidicbiooxidation effluent liquid to the thiocyanate leach solution, theacidic bio-oxidation effluent liquid comprising a concentration offerric iron that is larger than the concentration of the dissolvedferric iron in the feed of the thiocyanate leach solution.
 29. Themethod of claim 1, comprising, prior to the thiocyanate leaching,preparing the mineral material, the preparing comprising pressureoxidizing under acidic conditions feed material, the feed materialcomprising sulfide minerals that are decomposed during the pressureoxidizing.
 30. The method of claim 29, wherein the feed materialcomprises at least 2 weight percent sulfide sulfur.
 31. The method ofclaim 30, wherein the feed material comprises at least one of (i) an orecomprising at least 2 weight percent sulfide sulfur and (ii) a sulfideconcentrate made from the ore.
 32. The method of claim 1, comprising,prior to the thiocyanate leaching, preparing the mineral material, thepreparing comprising acid leaching feed material with an acidic leachsolution, the feed material comprising copper and during the acidleaching at least a portion of the copper is dissolved into the acidicleach solution, thereby removing soluble copper from the feed materialprior to the thiocyanate leaching.
 33. The method of claim 32, whereinthe copper-containing feed material comprises at least 200 parts permillion by weight of the copper that is dissolved into the acidic leachsolution during the acid leaching.
 34. The method of claim 1, whereinthe mineral material comprises at least one of (i) an ore comprising atleast 0.5 weight percent sulfide sulfur and less than 2 weight percentsulfide sulfur and (ii) a concentrate prepared from the ore.
 35. Themethod of claim 34, wherein the ore comprises no more than 1.5 weightpercent sulfide sulfur.
 36. The method of claim 1, wherein theincreasing a concentration of the dissolved ferric iron in thethiocyanate leach solution comprises adding dissolved ferric iron to thethiocyanate leach solution.
 37. The method of claim 36, wherein theadding dissolved ferric iron comprises adding acidic bio-oxidationeffluent liquid to the thiocyanate leach solution.
 38. The method ofclaim 1, wherein the increasing a concentration of the dissolved ferriciron in the thiocyanate leach solution comprises oxidizing to ferricform at least a portion of the dissolved ferrous iron in the thiocyanateleach solution.
 39. The method of claim 38, wherein the oxidizingcomprises introducing an oxidant into the thiocyanate leach solution.40. The method of claim 39, wherein the oxidant comprises a persulfate.41. The method of claim 39, wherein the oxidant comprises a componentselected from the group consisting of persulfuric acid, peroxide,manganese dioxide, ozone, a halogen and a hypochlorite.
 42. The methodof claim 38, wherein the oxidizing comprises contacting the thiocyanateleach solution with oxidizing gas.
 43. The method of claim 42, whereinthe oxidizing comprises contacting the thiocyanate leach solution withoxygen gas and a component selected from the group consisting of sulfurdioxide gas, a bisulfite and a metabisulfite.
 44. A method forseparating precious metal from a heap initially comprising preciousmetal-containing mineral material feed in particulate form, the methodcomprising: pretreating the heap, the pretreating comprising applying anacidic feed of pretreatment solution to the heap and percolating thepretreatment solution through the heap; after the pretreating,thiocyanate leaching the heap, the thiocyanate leaching comprisingapplying an acidic feed of thiocyanate leach solution to the heap andpercolating the thiocyanate leach solution through the heap, therebydissolving into the thiocyanate leach solution in the form of preciousmetal-thiocyanate complex at least a portion of the precious metal fromthe heap, wherein the feed of the thiocyanate leach solution comprisesdissolved ferric iron and dissolved thiocyanate at a molar ratio ofdissolved ferric iron to dissolved thiocyanate of at least
 2. 45. Themethod of claim 44, wherein the mineral material comprises at least 0.1weight percent soluble copper that dissolves into the pretreatmentsolution during the pretreating.
 46. A method for separating preciousmetal from a precious metal-containing mineral material feed in which atleast a portion of the precious metal is locked within one or moresulfide minerals, the method comprising: bio-oxidizing a heap initiallycomprising the mineral material feed in particulate form, during thebio-oxidizing a first portion of sulfide sulfur is oxidized; and afterthe bio-oxidizing, thiocyanate leaching the heap, the thiocyanateleaching comprising applying an acidic feed of thiocyanate leachsolution to the heap and percolating the thiocyanate leach solutionthrough the heap, the feed of the thiocyanate leach solution comprisingdissolved thiocyanate and during the leaching precious metal isdissolved from the heap into the thiocyanate leach solution in the formof precious metal-thiocyanate complex; wherein during the thiocyanateleaching, a second portion of sulfide sulfur is oxidized, the ratio ofthe second portion of sulfide sulfur to the first portion of sulfidesulfur being at least as large as 1:10.
 47. The method of claim 46,wherein the ratio of the second portion of sulfide sulfur to the firstportion of sulfide sulfur is at least as large as 1:4.
 48. A method forremoving precious metal from a mineral material feed comprising preciousmetal locked in one or more sulfide mineral, the method comprising:pressure oxidizing the mineral material feed in particulate formslurried in an aqueous liquid to decompose at least a portion of thesulfide mineral; recovering from the pressure oxidizing solid residueand acidic liquid effluent, the solid residue comprising at least aportion of the precious metal from the mineral material feed; andthiocyanate leaching at least a portion of the solid residue with anacidic thiocyanate leach solution to dissolve into the thiocyanate leachsolution in the form of precious metal-thiocyanate complex at least aportion of the precious metal from the solid residue.
 49. The method ofclaim 48, wherein the pressure oxidizing is conducted at a temperatureof at least 160° C. and an oxygen gas overpressure of at least 10 psi(68.9 kPa).
 50. The method of claim 49, wherein: the mineral materialfeed comprises a nonferrous nonprecious metal; the pressure oxidizingcomprises dissolving at least a portion of the nonferrous nonpreciousmetal into aqueous liquid, whereby the liquid effluent comprises aportion of the nonferrous nonprecious metal in solution; and separatingfrom the liquid effluent at least a portion of the nonferrousnonprecious metal, wherein the mineral material feed comprises at least1 weight percent of the nonferrous nonprecious metal that dissolves intothe aqueous liquid during the pressure oxidizing and that is thereafterseparated from the liquid effluent during the separating.
 51. A methodfor separating precious metal from a mineral material comprising theprecious metal and at least one copper-containing mineral, the methodcomprising: first leaching a feed of the mineral material with an acidicfirst leach solution to selectively leach copper into the first leachsolution relative to the precious metal; after the first leaching,second leaching the mineral material with an acidic second leachsolution comprising dissolved thiocyanate and dissolved ferric iron, andduring the second leaching at least a portion of the precious metal fromthe mineral material is dissolved into the second leach solution in theform of precious metal-thiocyanate complex; wherein a feed of the secondleach solution supplied to the second leaching comprises a molar ratioof the dissolved ferric iron to the dissolved thiocyanate of at least 2.52. The method of claim 51 wherein the feed of the mineral materialcomprises at least 0.01 weight percent copper that dissolves into thefirst leach solution during the first leaching.
 53. The method of claim52, wherein the first leaching comprises pressure oxidizing the feed ofthe mineral material in the presence of oxygen gas.
 54. A method forseparating precious metal from a mineral material containing theprecious metal, the method comprising: acidic leaching the mineralmaterial with a thiocyanate leach solution comprising dissolvedthiocyanate and dissolved ferric iron, wherein during the leaching atleast a portion of the precious metal from the mineral material isdissolved into the thiocyanate leach solution in the form of preciousmetal-thiocyanate complex; wherein, a feed of the thiocyanate leachsolution supplied to the leaching is at an acidic pH, comprises aconcentration of the dissolved ferric iron of at least 0.1 mole perliter and comprises a molar ratio of dissolved ferric iron to dissolvedthiocyanate of at least
 2. 55. The method of claim 54, wherein the feedof the thiocyanate leach solution comprises a concentration of dissolvedthiocyanate in a range of from 0.001 mole per liter to 0.03 mole perliter.
 56. A method for separating precious metal from a mineralmaterial containing the precious metal, the method comprising: acidicleaching the mineral material with a thiocyanate leach solutioncomprising dissolved thiocyanate and dissolved ferric iron, whereinduring the leaching at least a portion of the precious metal from themineral material is dissolved into the thiocyanate leach solution in theform of precious metal-thiocyanate complex; wherein, a feed of thethiocyanate leach solution is at an acidic pH and comprises a molarratio of dissolved ferric iron to dissolved thiocyanate of at least 7.57. The method of claim 56, wherein the molar ratio is at least
 10. 58.The method of claim 56, wherein the feed of the thiocyanate leachsolution comprises a concentration of dissolved thiocyanate in a rangeof from 0.0001 mole per liter to 0.03 mole per liter.
 59. A method forrecovering precious metal from mineral material containing the preciousmetal, the method comprising: thiocyanate leaching the mineral materialwith a thiocyanate leach solution to dissolve at least a portion of theprecious metal from the mineral material into the leach solution in theform of precious metal-thiocyanate complex; introducing dissolvedcyanide into the thiocyanate leach solution and transferring at least aportion of the precious metal from the precious metal-thiocyanatecomplex to precious metal-cyanide complex; removing from the thiocyanateleach solution at least a portion of the precious metal transferred tothe precious metal-cyanide complex.
 60. The method of claim 59, wherein,during the introducing and the transferring, the thiocyanate leachsolution comprises a molar ratio of dissolved cyanide to dissolvedthiocyanate that does not exceed 1:4.
 61. The method of claim 59,wherein the removing comprises separating from the thiocyanate leachsolution at least a portion of the precious metal in the form of theprecious metal-cyanide complex.
 62. The method of claim 59, wherein thethiocyanate leach solution is at an acidic pH during the thiocyanateleaching, the introducing, the transferring and the removing.
 63. Amethod for recovering precious metal from a thiocyanate leach solutionin which precious metal is dissolved in the form of preciousmetal-thiocyanate complex, the method comprising: transferring at leasta portion of the precious metal from the precious metal-thiocyanatecomplex in the thiocyanate leach solution to precious metal-cyanidecomplex in the thiocyanate leach solution; and removing from thethiocyanate leach solution at least a portion of the precious metaltransferred to the precious metal-cyanide complex.
 64. A method forseparating precious metal from a mineral material containing theprecious metal, the method comprising: leaching the mineral materialwith an acidic thiocyanate leach solution to dissolve into thethiocyanate leach solution in the form of precious metal-thiocyanatecomplex at least a portion of the precious metal; and preparing anacidic feed of the thiocyanate leach solution and supplying the feed ofthe thiocyanate leach solution to the leaching, the preparing comprisingdissolving cyanide in an aqueous liquid and after the dissolvingconverting the cyanide to dissolved thiocyanate.
 65. The method of claim64, wherein the feed of the thiocyanate leach solution comprises aconcentration of dissolved thiocyanate in a range of from 0.001 to 0.03mole per liter.